Here's a hint it's in the name of the machine, and it's not the word oven. Microwave ovens use microwaves. This spectrum ranges from radio waves and microwaves, to infra-red, visible light, ultraviolet, and x-rays.
Now when you press start on your microwave oven, it fires a blast of microwaves at your food. The energy in the microwaves excites water molecules in the food. The excited water molecules vibrate back and forth producing heat. And this heat cooks your food.
The great thing about microwaves is that they are at the right frequency to excite water molecules, but don't vibrate things like metal or plastic. Which is a good thing, because who wants to eat cooked plastic?
Critical thinking challenge: What else moves back and forth to produce heat? What is the name of that force that produces heat?