Percy Spencer created the first practical microwave oven after noticing that radiation from a magnetron melted a chocolate bar in his pocket.
Spencer was an engineer at Raytheon working with magnetrons, the vacuum tubes that generate microwave radiation for radar. After the chocolate observation, he tested the effect on other foods, including popcorn and an egg. Raytheon built the first microwave oven prototype, called the Radarange, in 1945.
The earliest machines were enormous and expensive compared with modern countertop ovens. Commercial models later became smaller and more affordable, allowing microwave cooking to spread into homes. Microwave ovens heat food mainly by causing polar molecules, especially water molecules, to move rapidly.
The chocolate-bar story is widely repeated, but the key invention was not simply discovering that microwaves could warm food. Spencer and Raytheon developed a controlled appliance around the magnetron’s heating effect.