Final answer:
A switch is used in a circuit to start or stop the flow of current, functioning like a valve to either block or allow current to pass through.
Step-by-step explanation:
To start and stop the flow of current around a circuit, you would insert a switch into the circuit. Unlike resistors, which limit the flow of charge in a circuit by adding resistance, a switch allows the current to be turned on or off without affecting the circuit's resistance. While a capacitor stores charge and blocks the flow of direct current while allowing alternating current to pass, and a conductor allows the current to pass with minimal resistance, only a switch can open and close the circuit on command, functioning similarly to a valve in a water circuit. When open, the switch prevents current from passing through; when closed, it allows the current to flow unimpeded.