The microwave was invented when Percy Spencer noticed a chocolate bar in his pocket melted nearby an active radar

Cooking food with microwaves was discovered accidentally in the 1940s. Percy Spencer, a self-taught engineer, was building magnetrons for radar sets with the company Raytheon. He was working on an active radar set when he noticed that a peanut chocolate bar he had in his pocket started to melt. The radar had melted his candy bar with microwaves. The first food to be deliberately cooked with Spencer's microwave was popcorn, and the second was an egg, which exploded in the face of one of the experimenters.^[1] To verify his finding, Spencer created a high density electromagnetic field by feeding microwave power into a metal box from which it had no way to escape. When food was placed in the box with the microwave energy, the temperature of the food rose rapidly.

http://en.wikipedia.org/wiki/Microwave_oven#History