Superheating and Microwave ovens

I think the following information is amusing, interesting, and potentially useful at the same time -- a rare combination.

If you heat water in a very smooth dish in a microwave, it may become heated way past the boiling point without actually starting to boil. Then when you put instant coffee or a tea bag in it... watch out. Very vigorous boiling starts instantaneously, possibly injuring you with spray.

In other words:

If one litre of water is superheated by only 1 °C (ie if it is heated to 101 °C without boiling), it is in an unstable state, and it can suddenly produce about 3 litres of steam. The rapid production of a substantial quantity of steam within the bulk of the water will cause it to boil vigorously and possibly to appear to explode. The result is boiling water flying at speed out of the container. [University of Southern Wales]

The reason it can happen:

In a microwave oven, the water is usually hotter than the container, whereas parts of the kettle or saucepan are usually hotter than the water. Further, the surfaces of some containers used in microwave ovens may be very smooth, almost at a molecular scale, whereas this is not true for kettles or saucepans.

Microwave ovens heat the water directly: the microwaves pass through the container and the water, and the water itself absorbs energy from them. In a kettle or saucepan, the container itself (saucepan) or a heating element (some kettles) is hotter than the water. The hottest points cause a small amount of local superheating, boiling is initiated here, and this then stirs the water. [same source]

See a movie of it happening and more information here.