Metal reflects microwaves and its free electrons can move in response to the oven's rapidly changing electric field. Usually a smooth metal surface simply reflects much of the radiation. Problems can occur at sharp points, thin edges or small gaps. Charge can become concentrated there, producing a very strong electric field. If the field becomes strong enough, it can ionise the surrounding air and create a spark, or electrical discharge. Thin metal can also heat rapidly. This is why microwave instructions warn against unsuitable metal objects. The important physics is the behaviour of charges in conductors and strong electric fields.

The effect becomes easier to notice when conditions change. Temperature, pressure and particle motion can alter how quickly processes happen or how matter behaves. Food storage, refrigeration, cooking and weather all give everyday ways to test the same particle ideas without needing specialist equipment.

At the microscopic level, the important particles are electrons. Moving electrons from one material to another leaves an imbalance of charge, and charged objects create electric fields around them. Those fields can attract or repel other charges without contact. In electronic sensors, very small changes in charge or capacitance can therefore be turned into a measurable electrical signal.