I have been using microwave oven lately to prepare hot water for coffee and tea. Until, someone told me it is dangerous. The liquid might undergo the phenomenon known as superheating, which creates an explosion like action when taken out. The hot liquid rushing out of the container is the dangerous thing. It will scorch every flesh on its path.
What is superheating? Water should boil at 100°C. However, sometimes the expectation never happens. The water or other homogeneous liquid, like tea and coffee, might reached 100°C or go beyond without bubble formation. There are times when the liquid surface tension (force) is enough to suppress it. Then the liquid bursts suddenly when disturbed by taking it off the oven or dropping any object in it.
How to prevent?
1) Never use any vessel with very smooth surface. Tiny air packets usually clung to rough or scratchy surface. The air packets help initiate bubble formation.
2) Avoid heating liquid that has been left undisturbed for a considerable time.Perhaps it has been degassed and has a greater chance of superheating.
3) Avoid reheating of previously heated water. Repeated heating encourages degassing, making it prone to superheating.
4) Drop a clean bamboo or wooden stick to water. It provides sufficient amount of air pockets and ensures bubble formation.
Water over a gas or electric stove may also superheat.