It was actually a scientist who made the first microwave.

They were doing experiments with hamsters with cryogenics, and warming up the frozen hamsters with hot paddles. It didn't work that well, and the scientist felt bad for the hamsters.

So, he built the first microwave to warm the hamsters more evenly and 'humanely'.

That's right. The first thing cooked in a microwave was literally a hamster.

EDIT:

First Desktop microwave that matches what we consider a microwave today, I should have said. My apologies.

References:

A Smith, J Lovelock, A Parkes, 1954: Resuscitation of Hamsters after Supercooling or Partial Crystallization at Body Temperatures Below 0° C.. Nature 173, 1136–1137

R K Andjus, J E Lovelock, 1955: Reanimation of rats from body temperatures between 0 and 1° C by microwave diathermy. The Journal of Physiology, 128.

Lovelock, J E, Smith A U, 1959, Heat transfer from and to animals in experimental hypothermia and freezing. Annals of the New York Academy of Sciences, 80: 487-499.