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Heating Water in a Microwave Safety. It can be dangerous.

Microwave
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Subject: Heating Water in a Microwave

I feel that the following is information that any one who uses a microwave oven to heat water should be made aware of.
About five days ago, my 26 year old son decided to have a cup of instant coffee.

 He took a cup of water and put it in the microwave to heat it up (something that he had done numerous times before). I am not sure how long he set the timer for but he told me he wanted to bring the water to a boil. When the timer shut the oven off, he removed the cup from the oven. As he looked into the cup he noted that the water was not boiling but instantly the water in the cup "blew up" into his face. The cup remained intact until he threw it out of his hand but all the water had flew out into his face due to the buildup of energy.

His whole face is blistered and he has 1st  and 2nd degree burns to his face which may leave scarring.
He also may have lost partial sight in his left eye.

 While at the hospital, the doctor who was attending to him stated that this a fairly common occurrence and water (alone) should never be heated in a microwave oven. If water is heated in this manner, something should be placed in the cup to diffuse the energy such as a wooden stir stick, tea bag, etc. It is however a much safer choice to boil the water in a tea kettle.

Here is what our science teacher has to say on the matter: "Thanks for the microwave warning.
I have seen this happen before. It is caused by a phenomenon known as super heating. It can occur anytime water is heated and will particularly occur if the vessel that the water is heated in is new.
What happens is that the water heats faster than the vapor bubbles can form.

 If the cup is very new then it is unlikely to have small surface scratches inside it that provide a place for the bubbles to form. As the bubbles cannot form and release some of the heat that has built up, the liquid does not boil, and the liquid continues to heat up well past its boiling point. What then usually happens is that the liquid is bumped or jarred, which is just enough of a shock to cause the bubbles to rapidly form and expel the hot liquid. The rapid formation of bubbles is also why a carbonated beverage spews when opened after having been shaken


About five days ago, my 26 year old son decided to have a cup of instant coffee.

 He took a cup of water and put it in the microwave to heat it up (something that he had done numerous times before). I am not sure how long he set the timer for but he told me he wanted to bring the water to a boil. When the timer shut the oven off, he removed the cup from the oven. As he looked into the cup he noted that the water was not boiling but instantly the water in the cup "blew up" into his face. The cup remained intact until he threw it out of his hand but all the water had flew out into his face due to the buildup of energy.

His whole face is blistered and he has 1st  and 2nd degree burns to his face which may leave scarring.
He also may have lost partial sight in his left eye.

 While at the hospital, the doctor who was attending to him stated that this a fairly common occurrence and water (alone) should never be heated in a microwave oven. If water is heated in this manner, something should be placed in the cup to diffuse the energy such as a wooden stir stick, tea bag, etc. It is however a much safer choice to boil the water in a tea kettle.

Here is what our science teacher has to say on the matter: "Thanks for the microwave warning.
I have seen this happen before. It is caused by a phenomenon known as super heating. It can occur anytime water is heated and will particularly occur if the vessel that the water is heated in is new.
What happens is that the water heats faster than the vapor bubbles can form.

 If the cup is very new then it is unlikely to have small surface scratches inside it that provide a place for the bubbles to form. As the bubbles cannot form and release some of the heat that has built up, the liquid does not boil, and the liquid continues to heat up well past its boiling point. What then usually happens is that the liquid is bumped or jarred, which is just enough of a shock to cause the bubbles to rapidly form and expel the hot liquid. The rapid formation of bubbles is also why a carbonated beverage spews when opened after having been shaken

 

Superheated Water

March 10, 2004
Lou Ann Jopp
Regional Extension Educator, Food Science
Extension Regional Center, St. Cloud
Phone: (320) 203-6058 or (888) 241-4591
Email: joppx001@umn.edu

Superheated water is extremely dangerous...people have been severely injured by such water

There have been reports of serious skin burns and scalding injuries around people’s hands and faces as a result of hot water erupting out of a cup after it had been over-heated in a microwave oven. Overheating of water in a cup can result in superheated water (past its boiling temperature) without appearing to boil. Superheating occurs if water is heated in a container that does not assist the formation of bubbles, which is a visual sign of boiling. Glass containers are the most likely to superheat water because their surfaces have few or no defects. The presence of slight defects, dirt, or other impurities usually help the water boil because bubbles will form on these imperfections. Adding materials such as instant coffee or sugar before heating greatly reduces the possibility of super heating.

Water does not always boil when it is heated above its normal boiling temperature of 100°C or 212°F.

Water can always evaporate into dry air, but it normally only does so at its surface. When water molecules leave the surface faster than they return, the quantity of liquid water gradually diminishes. That is ordinary evaporation. However, when water is heated to its boiling temperature, it can begin to evaporate not only from its surface, but also from within. If no bubbles are leaving the surface of the water as it is being heated to the boiling point, or above, steam bubbles may be forming inside the hot water. Water molecules then can evaporate into that steam bubble and make it grow larger and larger. When the water is disturbed in some way, it will boil violently. This may happen by inserting a fork or spoon into the water or striking the bottom of the container – and an explosion follows. When water is sufficiently superheated, all that is needed is just a single “seed” bubble to start an explosion and empty the container completely. This situation becomes even worse if the top surface of the water is “sealed” by a thin layer of oil or fat so that normal evaporation cannot occur.

Mild superheating happens fairly often and we rarely think much about it as we sponge up the spilled liquid inside the microwave oven. Severe superheating is less common but is a very dangerous phenomenon.

What Can Consumers Do to Avoid Super-Heated Water?

First: Follow the precautions and recommendations found in the microwave oven instruction manuals, specifically the heating time.

  • Do not use excessive amounts of time when heating water or liquids in the microwave oven.
  • Determine the best time setting to heat the water just to the desired temperature and use that time setting regularly.

If you really need to boil water, be very careful with it after microwaving or boil it on a stovetop instead. When you heat water on the stove, the hot spots at the bottom of the pot or defects in the pot bottom usually assist steam bubble formation so that boiling occurs soon after the boiling temperature is reached. Cooking water too long on a stovetop means that some of it boils away, but doing the same in a microwave oven may mean that it becomes dangerously superheated. Just a reminder that boiling water is a hazard for children even without superheating.

Second: Handle liquids that have been heated in a microwave oven with respect.

  • Do not remove a liquid the instant the oven stops. If the water was bubbling
    spasmodically or not at all despite heavy heating, it may be superheated and deserves particular respect. But even if you see no indications of superheating, it takes no real effort to be careful. If you cooked the water or any other liquid long enough for it to reach boiling temperature, let it rest for a minute per cup before removing it from the microwave.
  • Never put your face or body over the container and keep the container at a safe
    distance when you add things to it for the first time: powdered coffee, sugar, a
    teabag, or a spoon. The spontaneous bubbling that occurs when you add something to microwave-heated water is the result of such mild superheating. It is far better to have the liquid boil violently while it is inside the microwave oven than when it is outside on your counter and can splatter all over you.

Finally: This is not meant to scare you away from using your microwave oven or from heating water in it. It is intended to show you that there is a potential hazard that you can avoid. Microwave ovens are wonderful devices as long as you use them properly.

“Using them properly” means not heating liquids too long in smooth-walled containers

 

About Microwaves

Microwaves are used to detect speeding cars, to send telephone and television communications, and to treat muscle soreness. Industry uses microwaves to dry and cure plywood, to cure rubber and resins, to raise bread and doughnuts, and to cook potato chips. But the most common consumer use of microwave energy is in microwave ovens.

The Food and Drug Administration (FDA) has regulated the manufacture of microwave ovens since 1971. On the basis of current knowledge about microwave radiation, the Agency believes that ovens that meet the FDA standard and are used according to the manufacturer's instructions are safe for use.

What is Microwave Radiation?

Microwaves are a form of "electromagnetic" radiation; that is, they are waves of electrical and magnetic energy moving together through space. Electromagnetic radiation ranges from the energetic x-rays to the less energetic radio frequency waves used in broadcasting. Microwaves fall into the radio frequency band of electromagnetic radiation. Microwaves should not be confused with x-rays, which are more powerful.

Microwaves have three characteristics that allow them to be used in cooking: they are reflected by metal; they pass through glass, paper, plastic, and similar materials; and they are absorbed by foods.

Cooking with Microwaves

Microwaves are produced inside the oven by an electron tube called a magnetron. The microwaves are reflected within the metal interior of the oven where they are absorbed by food. Microwaves cause water molecules in food to vibrate, producing heat that cooks the food. That's why foods high in water content, like fresh vegetables, can be cooked more quickly than other foods. The microwave energy is changed to heat as it is absorbed by food, and does not make food “radioactive” or "contaminated."

Although heat is produced directly in the food, microwave ovens do not cook food from the "inside out." When thick foods are cooked, the outer layers are heated and cooked primarily by microwaves while the inside is cooked mainly by the conduction of heat from the hot outer layers.

Microwave cooking can be more energy efficient than conventional cooking because foods cook faster and the energy heats only the food, not the whole oven compartment. Microwave cooking does not reduce the nutritional value of foods any more than conventional cooking. In fact, foods cooked in a microwave oven may keep more of their vitamins and minerals, because microwave ovens can cook more quickly and without adding water.

Glass, paper, ceramic, or plastic containers are used in microwave cooking because microwaves pass through these materials. Although such containers can not be heated by microwaves, they can become hot from the heat of the food cooking inside. Some plastic containers should not be used in a microwave oven because they can be melted by the heat of the food inside. Generally, metal pans or aluminum foil should also not be used in a microwave oven, as the microwaves are reflected off these materials causing the food to cook unevenly and possibly damaging the oven. The instructions that come with each microwave oven indicate the kinds of containers to use. They also cover how to test containers to see whether or not they can be used in microwave ovens.

FDA recommends that microwave ovens not be used in home canning. It is believed that neither microwave ovens nor conventional ovens produce or maintain temperatures high enough to kill the harmful bacteria that occur in some foods while canning.

Microwave Oven Safety Standard

The Food and Drug Administration (FDA) has the responsibility for carrying out an electronic product radiation control program mandated by the Electronic Product Radiation Control provisions of the Food Drug and Cosmetic Act. Through it's Center for Devices and Radiological Health, FDA sets and enforces standards of performance for electronic products to assure that radiation emissions do not pose a hazard to public health.

A Federal standard limits the amount of microwaves that can leak from an oven throughout its lifetime to 5 milliwatts (mW) of microwave radiation per square centimeter at approximately 2 inches from the oven surface. This limit is far below the level known to harm people. Microwave energy also decreases dramatically as you move away from the source of radiation. A measurement made 20 inches from an oven would be approximately one one-hundredth of value measured at 2 inches.

The standard also requires all ovens to have two independent interlock systems that stop the production of microwaves the moment the latch is released or the door opened. In addition, a monitoring system stops oven operation in case one or both of the interlock systems fail. The noise that many ovens continue to make after the door is open is usually the fan. The noise does not mean that microwaves are being produced. There is no residual radiation remaining after microwave production has stopped. In this regard a microwave oven is much like an electric light that stops glowing when it is turned off.

All ovens must have a label stating that they meet the safety standard. In addition, FDA requires that all ovens have a label explaining precautions for use. This requirement may be dropped if the manufacturer has proven that the oven will not exceed the allowable leakage limit even if used under the conditions cautioned against on the label.

To make sure the standard is met, FDA tests microwave ovens in commercial establishments, dealer and distributor premises, manufacturing plants, and its own laboratories. FDA also evaluates manufacturers' radiation testing and quality control programs.

Although FDA believes the standard assures that microwave ovens do not present any radiation hazard, the Agency continues to reassess its adequacy as new information becomes available.

Microwave Ovens and Health

Much research is under way on microwaves and how they might affect the human body. It is known that microwave radiation can heat body tissue the same way it heats food. Exposure to high levels of microwaves can cause a painful burn. The lens of the eye is particularly sensitive to intense heat, and exposure to high levels of microwaves can cause cataracts. Likewise, the testes are very sensitive to changes in temperature. Accidental exposure to high levels of microwave energy can alter or kill sperm, producing temporary sterility. But these types of injuries - burns, cataracts, temporary sterility - can only be caused by exposure to large amounts of microwave radiation, much more than the 5mW limit for microwave oven leakage.

Less is known about what happens to people exposed to low levels of microwaves. Controlled, long-term studies involving large numbers of people have not been conducted to assess the impact of low level microwave energy on humans. Much research has been done with experimental animals, but it is difficult to translate the effects of microwaves on animals to possible effects on humans. For one thing, there are differences in the way animals and humans absorb microwaves. For another, experimental conditions can't exactly simulate the conditions under which people use microwave ovens. However, these studies do help us better understand the possible effects of radiation.

The fact that many scientific questions about exposure to low-levels of microwaves are not yet answered require FDA to continue to enforcement of radiation protection requirements. Consumers to take certain common sense precautions.

Have Radiation Injuries Resulted from Microwave Ovens?

There have been allegations of radiation injury from microwave ovens, but none as a direct result of microwave exposure. The injuries known to FDA have been injuries that could have happened with any oven or cooking surface. For example, many people have been burned by the hot food, splattering grease, or steam from food cooked in a microwave oven.

Ovens and Pacemakers

At one time there was concern that leakage from microwave ovens could interfere with certain electronic cardiac pacemakers. Similar concerns were raised about pacemaker interference from electric shavers, auto ignition systems, and other electronic products. FDA does not specifically require microwave ovens to carry warnings for people with pacemakers. The problem has been largely resolved because pacemakers are now designed to be shielded against such electrical interference. However, patients with pacemakers may wish to consult their physicians if they have concerns.

Checking Ovens For Leakage

There is little cause for concern about excess microwaves leaking from ovens unless the door hinges, latch, or seals are damaged. In FDA's experience, most ovens tested show little or no detectable microwave leakage. If there is some problem and you believe your oven might be leaking excessive microwaves, contact the oven manufacturer, a microwave oven service organization, your state health department, or the nearest FDA office.

A word of caution about the microwave testing devices being sold to consumers: FDA has tested a number of these devices and found them generally inaccurate and unreliable. If used, they should be relied on only for a very approximate reading. The sophisticated testing devices used by public health authorities to measure oven leakage are far more accurate and are periodically tested and calibrated.

Tips on Safe Microwave Oven Operation

  • Follow the manufacturer's instruction manual for recommended operating procedures and safety precautions for your oven model.
  • Don't operate an oven if the door does not close firmly or is bent, warped, or otherwise damaged.
  • Never operate an oven if you have reason to believe it will continue to operate with the door open.
  • As an added safety precaution, don't stand directly against an oven (and don't allow children to do this) for long periods of time while it is operating.
  • Users should not heat water or liquids in the microwave oven for excessive amounts of time.

Erupted Hot Water Phenomena in Microwave Ovens

The FDA received reports in the past of serious skin burns or scalding injuries around people's hands and faces as a result of hot water erupting out of a cup after it had been over-heated in a microwave oven. Over-heating of water in a cup can result in superheated water (water heated past its boiling temperature), which does not appear to be boiling.

This type of phenomena occurs if water is heated in a clean cup. If foreign materials such as instant coffee or sugar are added before heating, the risk is greatly reduced. If superheating has occurred, a slight disturbance or movement such as picking up the cup, or pouring in a spoon full of instant coffee, may result in a violent eruption with the boiling water exploding out of the cup.

What Can Consumers Do to Avoid Super-Heated Water?

Users should follow the precautions and recommendations found in the microwave oven instruction manuals, specifically the heating time. Users should not use excessive amounts of time when heating water or liquids in the microwave oven. Determine the best time setting to heat the water to the desired temperature and use that time setting regularly.

Other Tips for Microwave Oven Use

  • Some ovens should not be operated when empty. Refer to the instruction manual for your oven.
  • Clean the oven cavity, the outer edge of the cavity, and the door with water and a mild detergent. A special microwave oven cleaner is not necessary. Do not use scouring pads, steel wool, or other abrasives.
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UNWISE MICROWAVE OVEN EXPERIMENTS
High Voltage in the Kitchen

  
 

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Danger: Coffee Explosion

You warm up a mug of water for a few minutes in the microwave oven. You take it out, then you dump in some powdered coffee, tea, sugar, etc...
 

DOOSH! The water explodes in roiling foam, spraying boiling water all over your bare skin, and sending you to the emergency ward. I hate it when that happens.
 

Heating up water or coffee in a microwave oven can be dangerous, especially if you use a ceramic mug or clean glassware. Water sometimes "explodes" because the oven heats it to a temperature that's far hotter than the normal boiling point. When this occurs, any tiny disturbance can trigger some violent boiling. The stored energy of the above-100C water is released as a steam explosion. This DOESN'T happen when water is boiled in a pot on the stove. The difference: a stove creates small hotspots on the bottom of the pot which are far above 100C degrees, and these hotspots continuously trigger a roiling boil which cools the rest of the water down to 100C.
 

Whenever there are bubbles of steam zipping up through the water, those bubbles provide some surfaces which allow the water to make more steam, and as steam is created, the water cools down to 100C. In fact, water can only "boil" at places where the water surface touches a gas. If there are no bubbles already formed, then "boiling" will only happen at the top surface of the water and not down within it. So, whenever you heat water on the stove, the extreme temperature at the bottom of the pot causes tiny bubbles to form. The boiling water fills those bubbles with steam. The roiling bubbles act to cool the water and keep its temperature at (or below) 100C/212F degrees.
 

Things are different in a microwave oven. The water gets hot but the container usually does not. There are no tiny "boiling-bubbles" triggered by a hot stove burner. Without those bubbles to cool it, the temperature of the water can rise far higher than 100C. We call this "superheated water."
 

Superheated water is just waiting for some sort of trigger which will let bubbles form and allow boiling to commence. If the water becomes hot enough, a few bubbles will appear near the top, but these quickly rise and burst, and the water isn't cooled much at all. Even if your mug of water is bubbling slightly, don't trust it, since its temperature has risen so high above 100C that bubbles are appearing spontaneously. If some unwitting victim should pour powder into the superheated water, this will carry thousands of tiny air bubbles into the water. Each of these micro-bubbles expands into a large steam bubble, and the result is a huge "explosion" of hot froth. It's just like dumping ice cream into rootbeer, but the froth can be so violent that the hot water sprays into the air.
 

Even more dangerous is to boil water TWICE in a microwave oven. Most containers have tiny scratches in their surfaces, and these crevices contain air. When you heat water, these tiny air pockets will provide a constant stream of "seed bubbles" which allow normal boiling to occur. However, the air in these tiny bubbles within the cracks quickly gets replaced by steam. The crevices still produce seed-bubbles, but if you turn off the oven and let the water cool, the steam in the cracks will collapse and vanish, and the crevices fill with water. The seed bubbles are gone. If you now turn the oven on again, the water will superheat. Boiling your coffee twice can erase the bubble "nucleation centers." If your luck is bad, the water will superheat to a very high temperature, then explode violently when a single huge steam bubble spontaneously appears. If that bubble should start out at the bottom of the container, the explosion can fling the entire volume of hot water upwards. A few people have reported that sometimes the explosion is so violent that it makes a sharp noise, and can even crack a glass container.
 

MOST DANGEROUS:
  - BOILING PLAIN WATER...
  - IN A CLEAN SHINY CONTAINER (MUG OR PYREX)...
  - BOILING IT MORE THAN ONCE (LET IT COOL BETWEEN BOILINGS)...
  - COOKING IT EXTRA LONG (STORES LOTS OF ENERGY IN SUPERHEATING)...
  - REMOVING IT IMMEDIATELY (NO CHANCE TO COOL DOWN)
  - DUMPING IN SUGAR, CREAMER, A TEABAG, ETC. (SUDDENLY ADDS SEED BUBBLES)
 

If you avoid the items on this list, you'll probably never see a "coffee explosion." On the other hand, the above list is a "recipe for disaster." DON'T BE TEMPTED TO FOLLOW IT. Instead, here's a simple, HAZARDOUS experiment to try. Wear safety goggles, and don't heat the water for an excessive amount of time.
 

Fill a clean mug about 1/3 full of clean water (DON'T FILL IT TO THE TOP!), then heat it for about five minutes in the microwave oven. Now carefully take it out and immediately plunk it firmly onto the tabletop (whack it hard, but not so hard that it breaks.) The boiling water will burst into froth. DON'T BURN YOURSELF! The superheated water acts almost like warm carbonated cola: if you strike the container, it will foam up instantly.
 

Another trick: heat up the water to boiling again, remove it from the oven, then immediately insert a dry wooden coffee-stirrer, or a wooden popcicle stick into the water. Foosh! The water boils violently. The dry wood contributes a layer of air to the water, and the air fills with steam and expands into a mass of hot foam.
 

Another: heat up the water again, then pour a little bit of warm tap water into the superheated water. The water suddenly boils violently! It turns out that the tap water is full of tiny bubbles. If you let the tap water stand around for half an hour before pouring it into the superheated water, all the tiny bubbles in the tap water will have risen and popped, and the bubble-free water won't trigger any violent boiling. And if you then dissolve some salt into your "bubble-free" tap water, again that water WILL trigger boiling, since the salt contributes invisibly small bubbles.
 

Hmmmm. I wonder if de-ionized distilled water in a REALLY CLEAN container will superheat even more than normal? (DANGER, SUPERHEATED WATER CAN BURST OUT OF THE MUG AND SCALD YOU!) I wonder what would happen if we used vacuum-degassed water, or if we put some dishwashing soap in the water...
 

SAFETY WARNING: Treat microwave-boiled water with respect. It can "explode" without warning. You can "defuse" it by CAREFULLY inserting a dry wooden stir-stick or toothpick in order to trigger boiling. Don't dump any sugar in a mug of superheated coffee, or the spewing foam *really* gets violent. Don't try to boil liquids more than once, since that removes the tiny bubbles on the container surfaces which act as boiling centers. If you're going to re-heat a previously heated mug of liquid, cook it with a wooden stir-stick or wood chopstick which allows it to boil normally. Always allow bubbling liquids to cool for several minutes before adding anything to them (or perhaps reach over and carefully drop in a dry toothpick or a wooden stir-stick to force them into normal boiling mode.)
PS
Certain types of foods have no bubbles inside, and these foods will superheat and "explode." For example, never cook a whole unbroken egg in a microwave oven. The explosion isn't just messy, sometimes it's violent enough to smash up the inside of your oven or tear off the door. Paste-like canned foods easily superheat since they're too thick to allow streams of tiny bubbles to form. Canned spaghetti sauce is famous for superheating and causing those "BOOMF" mini-explosions that spray the sauce all over the oven. (I wonder if there's any cure for the "Spaghetti-O explosions?" Maybe whip the stuff with a fork before cooking, so lots of air is added? Mix it with dry bread crumbs or other material that's full of air?)
 

CLASSICS

There are many other excellent microwave demos on other sites. Stand up a CD in your oven and nuke it for about five seconds. Or convert Marshmallow Peeps into monsterous mutants. Slice a grape almost in half and watch it emit a six inch blowtorch of flaming plasma. Make showers of sparks with steel wool. Swell a chunk of Ivory soap into a blob of crunchy snow. Gamble on racing grapes.
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Untried experiments

Generate a glob of soot from burning paint thinner. Replace the air within the soot ball with pure oxygen, or ozone, or nitrogen, or argon. Place it within an active microwave oven. Is a Ball Lightning plasmoid created?
 

Light a candle and place it in the oven. Does the RF energy make the candle flame grow huge? If you place various metal salts on the wick, will the colored candle flame absorb RF energy better? Or, try running a wire up through the candle so its tip is in the flame. Any effects? There are reports of "ball lightning" being generated from candles, burning toothpicks, and burning plastic in Microwave Ovens.
 

Partially inflate a balloon with argon. Release the argon to purge the bit of air that was in the balloon, then fill it with pure argon. Carefully insert a wire up into the balloon so the wire tip is near the center of the sphere. Tie off the balloon. Place it on a plate in a microwave oven and turn it on. This should create a 700 watt "plasma ball" effect. However, it might also pop the balloon instantly. The tip of the wire will probably be melted by the intense corona. Anyone for "Kirlian photography" which vaporizes the object being photographed? If the balloon pops instantly, try the same thing by using a plexiglas box. (note: glue fumes wreck the effect, so hold the plexiglas together with tape.)
 

Try the infamous Microwave Powered Water-Fueled Lawn Mower. Do huge pulses of EM really extract energy from a mysterious source within water? Dr. Graneau says that high current discharge through liquid water produces numerous anomalies. Laugh if you wish, but only the real world can supply the real answer. "Let the experiment be Made!"
More and weirder non-microwave experiments

 

  
 

Other microwave oven sites on the WWW:

 

Microwave oven Ball Lightning

Microwave oven chemistry experiments