Microwave Superheated Water
Microwaved water and other liquids do not always bubble when they reach the boiling point. They can actually get superheated and not bubble at all. The superheated liquid will bubble up out of the cup when it is moved or when something like a spoon or a tea bag is put into it, which can cause injury.
To prevent this from happening, do not heat any liquid for more than two minutes per cup. After heating, let the cup stand in the microwave for thirty seconds before moving it or adding anything into it.
This information can also be found in the front of the owner's manual that comes with each microwave.