An electrical system works a little like a plumbing system.
Power comes from the utility company into the service panel.
Figure B Figure C A light switch operates by opening and closing an electrical circuit. Upgrading or replacing a light switch is a simple project. Turn off the power at the circuit-breaker box, and remove the cover from the existing switch. Hold one lead against the bare ground wire, and touch the other lead to both of the black wires attached to the switch. If the power is off, go ahead and remove the switch by taking out the retaining screws.