NOTHING
With no food or liquid to absorb the microwaves, the magnetron (which is what makes the microwave function) ends up absorbing the microwaves instead, which can damage your microwave and even start a fire, according to the USDA. Make sure you don’t accidentally press ‘start’ without food or drink inside.
CUPS OF WATER
When plain water is heated in a microwave in a ceramic or glass container for too long, it can prevent bubbles from forming, which usually help cool the liquid down. The water becomes superheated; when you move the cup, the heat releases violently and erupts boiling water. To avoid this scalding risk, heat water only the minimum amount of time needed or heat it for longer in small cycles.
