About five days ago, my friend 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. Then 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 flown 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 may also have lost partial sight in his left eye.
While at the hospital, the doctor who was attending to him stated that this is a fairly common occurrence and water (alone) should never be heated in a microwave oven. If water is heated in this manner, something such as a wooden stir stick or a tea bag should be placed in the cup to diffuse the energy.
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 any time 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.