The part that spins on top of a microwave is called the "turntable" or "rotating plate." It is a circular glass or plastic plate that sits on a rotating mechanism at the bottom of the microwave's interior. The turntable rotates during cooking to ensure even distribution of the microwave energy, helping to cook or heat food more uniformly. This feature prevents hotspots and cold spots in the food, leading to better cooking results. Not all microwaves have a turntable, but it is a common feature in most modern microwave ovens.