system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it bad to have a microwave running with nothing inside it?

A friend tells me it's bad but offers no reason why.
gpt
A microwave heats food by generating lots of long wave electromagnetic waves (micro-waves!) which bounce around the chamber, bash into the food, interact with the molecules of the food and transfer the energy into heat to make your dinner hot.

The walls of the microwave are designed to bounce the rays off rather than absorb then, the theory being that they will eventually hit the food  to warm it up. If there is nothing inside, the rays will bounce around in the contained box and can potentially cause an arch and potentially burn the inside of the microwave, and can also bounce around and damage the electrical parts of the machine which will eventually damage it and brake it.