 Switches. Suppose that a lamp has two bulbs: one on the left, and one on the right. There is a switch which determines whether power will flow to the bulb on the left or the one on the right. If the power is on and the switch is set to the left, then the left bulb will be on, and the room will be illuminated. If the power is on and the switch is set to the right, then the right bulb will be on, and the room will be illuminated. If the power is off, then neither bulb will be on, and the room will be dark, no matter how the switch is set. To start, the power is off and the switch is set to the left. Filipa flips the switch to the right, and Phoebe turns on the power. The right bulb turns on, and the room is illuminated.