Here's a hint. It's really not an oven at all. Microwave ovens use microwaves. These include infra-red, visible light, ultraviolet and x-rays.
Press start on your microwave oven. This 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. This produces heat. And this heat cooks your food.
The great thing about microwaves is that they are at the right frequency to excite water molecules. They don't vibrate things like metal or plastic. Which is a good thing. Because who wants to eat cooked plastic?