A light switch controls the flow of electricity through a circuit—a complete, unbroken loop through which electric charges can move. When the light switch is on, these electric charges can move in an endless loop. This loop starts with a trip to the power company—actually to the power transformer near your home—where the charges pick up electric energy. They then flow through wires to the light switch, then to the light bulb where they deliver their electric energy, and finally back to the power company to obtain more energy. The same charges complete this loop over and over again. The loop is called a circuit.

But when you turn off the light switch, you open or break the circuit. One of the wires connecting the power company to the light bulb suddenly has a gap in it and the current of electric charges can no longer flow. The switch itself actually contains two separated wires and a mechanical device that connects them only when the switch is in its on position. The precise structure of the mechanical switching device differs from switch to switch, but the behavior is always the same: the switch disconnects the two wires—and thus breaks the circuit—whenever you turn the switch off.