Biting into aluminum foil can cause a sudden sharp pain if the foil comes into contact with a metal filling in the tooth. This pain is the result of an electric current.
Most metals have a tendency to give up electrons.
This transfer produces an electric current. Battery acid is the electrolyte in most batteries. When you bite into foil, your mouth becomes like an electric battery. The foil and the filling act as the posts of the battery, and your saliva is the electrolyte. Biting on the foil completes the circuit.