Microwaves were invented by accident while engineer Percy LeBaron Spencer was testing magnetrons.
Magnetrons are " vacuum tubes that produce microwave radiation and are used in radar systems," according to LiveScience. Spencer noticed that a chocolate bar in his pocket melted while he was testing them. He invented the microwave by building a metal box around them since microwaves can't pass through metal, and found that the result could heat food faster than ovens. He filed for a patent in 1945.
#10 An old choir director of mine lived next door to one of the guys who developed the earliest patents for home microwave ovens. They were approved and being marketed by the 70s, but he wouldn't have one in his house until the 90s because they still leaked too much radiation of some sort.