Heating water in a microwave oven can be hazardous to your health and safety.
There are stories of people who have put a cup of water in the microwave to bring it to a boil before adding tea or coffee, only to take it out of the oven and have the water erupt in their faces, causing serious injury.
Boiling Water
To understand how this happens we need to quickly review what boiling is. When water is heated to the boiling point the liquid water changes to gas, thereby expanding its volume greatly. Boiling is the expanding of the water molecules into the gas bubbles.
However, for these bubbles to form there must be something like bits of dust, a tea bag, or rough sides on the container for them to form on. If the container you use to heat the water is new, or highly glazed it might have very smooth sides, and no rough places on which bubbles can form. If it's very clean there might not even be bits of dust for the bubbles to form on.
Bubbles, Bubbles, Bubbles!
With no bubbles forming and rising to the top to break the surface and allow steam to escape, the water can continue to heat up past its boiling point to what is called a "superheated" state. The surface of the water literally holds the heat in, like a lid. When the water's surface is finally disturbed by moving the cup or putting a tea bag in, the superheated water instantly and explosively comes to a boil, throwing steam and very hot water all over.
If you plan to heat water in a microwave, put something nonmetallic in the cup, like a wooden stirring stick or a tea bag. To be even safer heat the water on a stove in a tea kettle, because this dangerous phenomenon does not occur on a regular stove.