Why can a microwave be metallic?

The exact dimensions of the metal lining in your microwave oven are 'tuned' to the frequency of the magnetometer to reflect the microwaves back and forth and create an even electromagnetic field inside by creating a 'standing wave'. Putting other metal in the oven will cause other reflections, which can bounce back into the magnetron (which created the microwaves) and cause an explosion. Also, the radio waves cause an electric current to run through metal, if two pieces of metal are close together, then this can result in the electric current jumping between them and causing an electric arc which looks a bit like lightning.