The microwave was invented after a tech noticed a radar they were working on melting a candy bar in their pocket. It was initially aimed at food preparation.
"A hamster is an acceptable size."
https://royalsocietypublishing.org/doi/10.1098/rspb.1956.005...
"Studies on golden hamsters during cooling to and rewarming from body temperatures below 0° C. III. Biophysical aspects and general discussion"