Good questionThere are two issues here: the food does not get heated, and there are arcing/sparksMost metals reflects microwave energy, including stainless steel and aluminiumHence the food will not get heated if you place it inside stainless steel or aluminium foil When two metal objects, such as a metal bowl and the metal cavity wall, are placed in close proximity and subjected to an intense field of microwave energy, arcing will resultArcing occurs because the air between the two metallic objects becomes electrically chargedThis ionized air becomes an electrical conductor, and electric current then leaps the gap like a small bolt of lightningAn arc in a microwave oven will continue, as long as the microwave energy is appliedAt the very least, this can cause marring or pitting of involved surfaces, and at worst, can burn a hole right through the cavity wall.
