Why should you avoid metal in a microwave?
Short answer
Metal reflects microwaves, and its shape and position can concentrate the electric field. That can cause sparks, uneven heating, or damage to the oven.
How it works
A microwave oven reflects microwaves from its metal walls so that food can absorb them. Water and other molecules in the food move in response and the food heats up. Metal does not absorb microwaves in the same way; it reflects them.
Sharp edges, thin metal, or the wrong placement of foil can concentrate the electric field and create sparks through the air. This is why metal containers and foil are generally avoided.
Key point
This does not mean every piece of metal is always dangerous. Unless the oven manual specifically allows an item, use cookware labeled as microwave-safe.