Microwaves are a kind of electromagnetic wave and as such can create electric currents in metals. Many of the microwaves are actually reflected by the metal and can reflect back onto the magnetron – the part of the oven that produces the microwaves, which can overheat and become damaged. When microwaves hit metal surfaces, free electrons (negatively charged particles) in the metal start to move around and the movement of this charge is how an electric current arises. Some of these electrons will move too much and will actually jump from the metal to the air which becomes temporarily ionised (charged). This can result in a phenomenon called ‘arcing’ where an electric spark similar to a flash of lightning is produced. Thin metal such as rims of mugs, produce more resistance to an electric current than thick metal, and so can become very hot. If sufficiently thin the metal can become so hot that it actually melts!