How does a microwave oven heat food without a flame?

A microwave oven heats food by making its molecules spin. Inside the box, a stream of microwaves, a kind of electromagnetic radiation, sets up an electric field that flips back and forth billions of times a second. Water, fats and many other molecules in food are lopsided, slightly positive at one end and slightly negative at the other, so they keep twisting to line up with the field. All that twisting turns into heat. This is called dielectric heating.

Household ovens use microwaves of about 2.45 gigahertz, with a wavelength of 12.2 centimetres. Despite the popular idea, food does not cook from the inside out: the microwaves only reach about a centimetre into most foods, and the middle of a thick dish warms up as heat creeps inward from that outer layer. That's why a big plate of leftovers can be scalding at the edge and cold in the centre.

The invention began with an accident. Percy Spencer, an engineer at the American company Raytheon, noticed that the microwaves from a radar set he was working on had started to melt a candy bar in his pocket. He tried popcorn next, then an egg, which exploded in a colleague's face. In 1947 Raytheon sold the first commercial microwave oven, the Radarange, a machine almost 1.8 metres tall.