Our county is conducting a science fair for all the local schools. Or if you do do it, do it with a microwave that no one will ever want to use again, because you will probably destroy the microwave if you run it for very long without any water inside. Also, if you do do it, make sure there's an adult around in case something catches on fire.
Microwaves work by shooting tiny waves called microwaves through food.
Microwaves are a lot like light waves, except that they're a lot bigger. When the microwaves go through the food, they make the water molecules flip up and down very quickly. Molecules other than water won't be affected by this much. If you run the microwave without anything in it, the microwaves aren't absorbed by anything except for the microwave itself. There are certain parts in the microwave that can be damaged by absorbing microwaves.