Though it was first developed as a military weapon, radar promised big advances in air and sea travel, weather forecasting — the list is long. In 1945, with the war winding down but radar work still moving forward, a researcher in the United States named Percy Spencer walked by a cavity magnetron while it was in use.

Spencer felt some heat, and noticed that the chocolate bar he’d been carrying in his pocket had melted into a sticky mess. Other researchers had already noticed that it was a bad idea to place certain things — lunch, for example — by the magnetron. Spencer, however, decided to investigate further.

He tried placing popcorn near the magnetron. The popcorn popped. Next Spencer tried an egg. It exploded in a colleague’s face. Something strange — and very interesting — was going on.

The microwaves produced by the magnetron were heating the water in Spencer’s food. It was this heat that melted the chocolate, popped the corn, and blew up the egg. Why should microwaves be good at heating water?

The water molecule, H2O, has two hydrogen atoms for every one oxygen atom. The two hydrogen atoms stick off the oxygen atom sort of like a boomerang. This boomerang shape affects the electrical properties of the water molecule, and makes it twist whenever it meets a microwave.

All this molecular movement heats up the water. That’s really all microwave ovens do. They use microwaves to twist — and thereby heat — water molecules.

By 1947, the Raytheon corporation (Spencer’s employer) had patented Spencer’s “radar range”. The first microwave ovens were ridiculously large and expensive. In the 1950s, other companies tried to sell the device as an “electronic oven”, but the price tag kept most people away. Finally, by around 1967, microwave ovens were small and cheap enough to start appearing in the average family’s home. By the 1980s, microwave ovens were everywhere, warming up burritos, popping corn and, of course, melting chocolate.

In our final chapter, we’ll learn some of the ins and outs of your microwave oven.