Microwave ovens work by emitting microwave radiation that causes water and fat molecules in food to rotate rapidly, generating heat through molecular friction. Percy Spencer discovered this effect accidentally in the 1940s while developing radar technology. Microwaves efficiently heat food from the inside out by exciting the polar molecules. However, long term effects of microwave radiation exposure are still being studied, as some research has linked microwaves to increased cancer risk and nutritional losses in cooked food.