Is it safe to put Metal in Microwave Oven?

It isn't necessarily bad to put metal in a microwave oven, but it can cause cooking problems or other trouble. Microwaves cause currents to flow in metals.

In a thick piece of metal, these currents won't cause problems for the metal. However, in thin pieces of metal, the currents may heat the metal hot enough to cause a fire.

Metallic decorations on fine porcelain tend to become hot enough to damage the porcelain. But even thick pieces of metal can cause problems because they tend to reflect the microwaves. That may cause cooking problems for the food nearby.