Makey Makey Classic works through the opening and closing circuits, just like any other button. Instead of the circuit being closed underneath your keyboard, the circuit is closed through the conductive objects you connect with alligator clips like your hand or your lunch or some tinfoil or lead pencil or playdoh.
Remember the principal of circuits a circuit must be closed for the electricity to flow. Under this principle, you make an extension from your computer to the outside object giving that outside objects the same property as a mouse or keyboard.
When you press down on your mouse or key on your keyboard you are close the connection and the electricity is connected and flows. So you extend the connection outside the mouse/keyboard of your computer by plugging in a makey makey. But it all about closed circuits.