313. How can microwaves heat something? Radio waves don't warm things very much.
The electric field of a microwave flips back and forth at just about the right frequency to have the largest effect on water molecules. The water molecules try to follow the reversing electric field and, in doing so, become hotter and hotter. Radio waves flip too slowly to have very much effect on water. Furthermore, the microwaves in an oven are far more intense than the radio waves that we're used to have around us so that common radio waves just don't do very much cooking.