Microwaving metal in a microwave oven, especially crinkled tin foil or formed metal like forks and containers, could cause a fire. When you put a metal shape in the middle of a stainless steel microwave oven, the metal is reflecting microwaves back and forth at a frenetic pace. All that amplified energy heats up and creates crackles and pops, which could ignite the contents of the microwave. Microwaves are meant to safely interact with liquids, fats, and sugars in food, not with other metals inside the oven. Microwaves have metal walls to confine the radiation to the inside of the oven.