The microwave has evolved into one of the most popular household appliances in the world, but few people know that it was invented entirely by accident. In 1945, Percy Spencer, an American self-taught engineer, was working in a lab testing magnetrons, the high-powered vacuum tubes inside radars. One day while working near the magnetrons that produced microwaves, Spencer noticed a peanut butter candy bar in his pocket had begun to melt — shortly after, the microwave oven was born.