Why can't you put metal in a microwave?

Dig into the science of how microwave ovens use electromagnetic waves to heat your food, and what you should avoid cooking in them.

In 1945, engineer Percy Spencer was standing near a RADAR device that produced high-intensity microwaves and noticed that his candy bar had melted. He then exposed popcorn kernels to the magnetron device, and sure enough, they popped. Soon after, the first microwave oven became available, using the very same technology. So, how does it work?