The magnetron produces microwaves at a frequency that reacts only with water molecules in food, which is why molecules of glass, plastic, or paper don’t react to the microwaves and they don’t heat up (except by conduction from the warm food). Metal objects reflect microwaves. This is why the sides of the oven are made of metal - to reflect the microwaves and keep the radiation in the oven. Even the glass door has a metal shield that reflects the microwaves. However, if you put a metal object into the microwave, it could reflect the waves back to the magnetron and do considerable damage.