Metal, including aluminum foil, bread ties and utensils, reflects microwave energy instead of letting it pass through. That builds up heat, causing sparks (aka, arcing).

A large sheet of very thin metal, like a big piece of aluminum foil, can in fact heat up extremely rapidly, becoming so hot that it could start to burn the microwave. So don’t do that.

But metal isn’t prohibited all the time. Newer microwave models are designed to be used with small amounts of metal. Be sure to check your manual before trying this, however.

Microwave ovens will be the second part of the blade architecture in final form,” he said. “So that’s it. Humans have been pouring metal in molds for 5,000 years.