A video of this demonstration is available at this link.

Turn on the Van de Graaff generator. As the dome charges, the charge is conducted through the stack of pie tins. Since they all have charge of the same sign, they repel each other, and one by one, they fly off the dome of the generator. Important! Always use the grounded wand to turn the generator on and off.

Demonstration 56.45 -- Sparks, describes how a Van de Graaff generator works. Briefly, a motor-driven belt carries charge that has been deposited on it via corona discharge from a row of needles at the bottom of the generator, below the drive pulley, up to a second row of needles at the top, inside the dome, where via corona discharge, the charge is deposited on the dome. The generator can thus charge its dome to very high potentials.

Resting on the dome of the Van de Graaff generator is a stack of three aluminum pie tins. Since the pie tins are conductive, as the dome of the generator charges, charge is distributed through the stack of pie tins. Since the dome and all the pie tins have charge of the same sign, they repel each other, and the pie tins fly off the dome one by one.