How does a microwave oven work?

Microwave ovens use high-energy electromagnetic waves (or
microwaves) to heat foods by energizing the food molecules, causing them to reverse
polarity millions of times per second.  The resulting friction generated by the
molecules heats the food.  No external source of heat is used in microwave
cooking.

The key component of a microwave oven,
and easily the most expensive part is a magnetron tube.  This element produces
electromagnetic waves called microwaves that are reflected by metal and absorbed by
moisture-containing substances such as
food.

Unlike ionizing radiation (such as X-rays
or gamma rays) that can alter molecular structure, non-ionizing microwaves have no
permanent effect on food molecules.

Unless
specific instructions are provided, metal should not be placed in microwave ovens. 
Metal prevents the microwaves from reaching foods, and may cause arcing between the
metal and oven walls, reducing the life or damaging the expensive magnetron
tube.

Microwaves occur between radio waves and
visible light in the electromagnetic spectrum.  In addition to use in residential and
commercial food ovens, microwaves are used in communication and radar systems, and have
many other commercial applications.