Which material should never be placed in the microwave due to its ability to repel microwaves?

Metal should never be placed in the microwave because it reflects microwaves instead of allowing them to pass through. When microwaves hit metal, they can create sparks and possibly even fires, as the energy cannot be absorbed effectively to heat the food. This is particularly dangerous for items that have sharp edges or points, as they can cause arcing, which is the term used for the sparks that might occur.

In contrast, plastic, paper, and glass are generally microwave-safe materials, as they allow microwaves to penetrate and heat the food without causing damage or creating hazardous conditions. While there are some exceptions regarding specific types of plastic (not all plastics are microwave-safe), paper can be used for heating food and glass containers are specifically designed for microwave use. This highlights why metal stands out as a material to be avoided in microwave cooking.