Metal should not be put in the microwave because it can cause arcing, sparks, and potentially a fire due to the way microwaves interact with metallic materials.
Microwaves work by emitting electromagnetic radiation that excites water molecules in food, causing them to vibrate and generate heat. Metal objects, however, contain free electrons that are easily moved by this electromagnetic field. When microwaves hit metal, these electrons are forced to move rapidly, creating an electric current. If the metal object has sharp edges or points, the concentration of electrons at these points becomes very high. This high concentration can lead to a phenomenon called 'arcing,' where electricity jumps from one part of the metal to another, creating sparks. In some cases, this arcing can ignite flammable materials inside the microwave, leading to a fire. Furthermore, thin pieces of metal, like aluminum foil, can heat up very quickly and potentially melt or cause damage to the microwave itself. While some metal containers are specifically designed and tested to be microwave-safe, it's generally best to avoid putting any metal in the microwave unless explicitly stated otherwise.
Always check the packaging of food containers before microwaving them. Look for a microwave-safe symbol, which usually indicates that the container is made of a material that won't react negatively to microwaves. If in doubt, transfer your food to a microwave-safe glass or plastic container.