Boiling water in the microwave can be dangerous. When you microwave water, it doesn't always bubble when it reaches the boiling point. The water can become superheated and not bubble at all. When the water then gets moved (such as when you go to get it out of the microwave), the superheated water will then bubble up out of the cup, often violently sending superheated water everywhere... including on you.

To keep this from happening and causing burns, do not heat any liquid in the microwave for more than two minutes per cup. And after heating liquids in the microwave, let them sit in the microwave for thirty seconds before moving them or disturbing them in any way.