Why does metal blow up your microwave?

Answer 1

Putting metal in the microwave does not cause it to explode, but it will likely break it, cause arcing, or even cause a fire. There is no shrapnel flying, the microwave just quits working. What usually happens when the microwave fails is that the antenna burns off of the magnetron. There is nothing to absorb the energy when you run a microwave empty or put metal in it, so it damages the magnetron that produces the microwave energy.

The radio waves in a microwave oven induce currents into metal and heat it up, especially if the metal is thin. This is how microwave pizzas cook the dough on the bottom. The little gray platform in the box that you set the Pizza on is a metallic film.

Charge will also tend to build up on any edges or points in the metal. If enough charge builds up, it can arc to nearby pieces of metal. Rolled up foil will arc unless it is rolled very tightly to reduce the pointy edges. Steel wool arcs rather spectacularly. A fork can sometimes arc, but usually the tines of the fork are too far apart and all the fork does is heat up.