The reason why metals or materials with metallic surfaces are highly discouraged from being used in microwaves is due to how microwaves interact with metals. Lets first talk about how microwaves work to heat food. A microwave generates electromagnetic waves of a particular wavelength (approximately 10 cm). Water molecules are efficient in absorbing electromagnetic radiation right around this wavelength. By absorbing this radiation, the microwaves increase the internal energy of the water molecules causing them to vibrate and rotate faster. This increased motion causes the water molecules to collide with each other and with other nearby molecules, generating heat in the process.

Metals behave vary differently because their atomic structure is very different from non-metals. Metals also absorb microwaves, but the electrons associated with metallic atoms are not constrained, unlike those associated with water molecules. This causes the metallic electrons to essentially move much faster than the electrons in the water molecules, resulting in more collisions. This causes the metal to heat up much more quickly and can easily reach extremely high temperatures hot enough to melt the metal in some cases. An additional problem resulting from having metal inside a microwave is that the electrons moving around the surface generate currents. Under certain conditions, these currents can give rise to electric discharges which are commonly manifested by sparks of light and popping sounds. These electric discharges, in addition to damaging the metal, can also damage the microwave itself.