It's practically an urban legend: a man heats water in the microwave for a cup of instant coffee. While taking the cup out of the microwave, he notices the water doesn't have any bubbles...at which point, the water boils up and explodes, giving him burns on his face and hands. Is there any truth to this myth?
Technically, and somewhat surprisingly, yes! Microwaves are specifically designed to heat water molecules and to heat them very rapidly. This can end up heating the water faster than it can turn into vapor, causing it to become superheated. Bubbles of water vapor can then quickly form when the water is jostled as the cup is taken out of the microwave. This makes it boil up, seeming to "explode," as all the vapor is suddenly released.
But in reality, the conditions that make this happen are relatively rare. Unless it's brand-spanking-new, the cups we use to heat water in the microwave likely have scratches that create an uneven surface where water vapor bubbles will form. Superheating will also only happen if the water is microwaved for an excessively long time, far longer than is actually needed to heat the water (2-3 minutes).
To insure yourself against a boil-up, you can put a wooden skewer or other non-metal utensil in the cup of water to help diffuse the heat and act as a surface for bubbles to form. You can also let the water cool down for a minute in the microwave before moving the cup or adding anything into it. Or you could boil water on the stove!
Has this "exploding water" phenomenon ever happened to you?
More Information on Exploding Water:
• Can Microwaved Water Explode? from Chow.com (with Mythbusters video!)
• Exploding Water in the Microwave from Steve Spangler Science
• Boil on Troubled Waters from Snopes