The reason you are advised not to put metal objects in a microwave oven is due to the way microwaves interact with metal. Microwaves work by emitting electromagnetic waves that excite water molecules and other polarized molecules in food, causing them to vibrate rapidly, generating heat and cooking the food.
When metal objects are placed inside a microwave, they can reflect or redirect the microwaves. The walls of a microwave oven are made of metal, but they are specifically designed with small holes or perforations that allow the microwaves to pass through and heat the food while keeping the radiation contained within the oven.
However, when you place a solid metal object inside a microwave, such as aluminum foil, utensils, or containers with metal components, these objects can create an effect called "arcing." Arcing is the phenomenon where electrical sparks are generated when the microwaves come into contact with the metal, causing a potentially hazardous situation.
The sparks can damage the interior of the microwave, including the walls and the magnetron (the device that generates the microwaves). Additionally, the intense heat generated by the electrical sparks can pose a fire risk and potentially damage the microwave or create a safety hazard.
To ensure safe and proper operation of a microwave, it's important to follow the manufacturer's guidelines and avoid putting any metal objects inside the oven. Use microwave-safe containers and utensils made of materials specifically designed for microwave use, such as glass, ceramic, or microwave-safe plastics.