Why does aluminium foil spark in the microwave?

Why does aluminium foil spark in the microwave?

The reason for the sparking is the photo-electric effect. The microwaves energize outer orbital electrons in metals and causes emission. Due to the conductive nature of the Al and insulting nature of the air (it's a dielectric), the electron charge can build. However, recombination of electrons into the orbitals is possible, but very slow compared to the discharge rate. The resulting charge becomes strong enough to ionize the air and create a conductive path to the walls of the microwave.

The electro-magnetic fields are highly dependent on the topology of the metal object. Where you have a sharp edge, there is a significant concentration of electric potential and therefore, the edge is more likely to discharge the electrical charge build-up.

Another example of this topological effect are tesla coils, where you'll often observe that folks place a rod on the torus to force the arc to originate that the end of the rod. The charging mechanism of electrons to the torus is completely different in this case, however.