My Microwave Doesn't Spark..? The inside of my microwave is metal, so why doesn't it spark when it's turned on?

The inside is a good electrical conductor, so there's an antinode in the electromagnetic field at the surface - the microwaves are shorted out, basically.

Besides, microwaves ovens are designed to cook food, not as spark generators.

More to the point to ask why some metal objects do spark inside the oven.

The way I understand it is this - microwaves are an electromagnetic wave, consisting of smoothly changing electric and magnetic waves, so that at some instant two points a distance apart may be at different voltages. If you put a metal object in, that distorts the electric field because points on a conductor must be at the same voltage. If it's got sharp points, the electric field can be very strong at the edge, strong enough to ionize air molecules. The inside of the microwave does not have sharp points.

Also, if you put in a wire loop, it can resonate like a tuning fork. I just tried a 2" loop of 16AWG copper with a 1/4" gap which was pretty spectacular.

BECAUSE IT IS ITS ANTENNA THAT RESONATE AT THAT MICROWAVE FREQUENCY.