https://www.scientificamerican.com/article/do-microwaves-cause-cancer/

In 1946, an engineer was working near a piece of equipment known as a magnetron which is part of a radar system when he noticed that the radar emission at microwave frequencies melted the snack bar he had stashed in his pocket. Legends disagree as to whether it was a peanut cluster bar or a chocolate one, but the fact remains that rather than just mourning the loss of his snack, he did a little investigating.

He and his coworkers realized that focused beams of microwave emission—that’s waves of energy at frequencies near the radio frequency regime—will cause polar or electrically asymmetric molecules like water in food to rotate. This rotation produces thermal energy (aka heat) which then can quickly and nearly uniformly heat the food.