Just think about it this way. You have a conductor(tin foil) on the top and bottom plates(cardboard) with something separating them(cotton balls). When you complete the circuit(push the plates together) the buzzer goes off.
Just think about it this way. You have a conductor(tin foil) on the top and bottom plates(cardboard) with something separating them(cotton balls). When you complete the circuit(push the plates together) the buzzer goes off.