The Loop That Powers Everything

A circuit is just a closed loop. Electrons leave one terminal of a power source, travel through components that do useful work, and return to the other terminal. Break the loop anywhere and everything stops. That's literally what a light switch does — it breaks the loop.

Every circuit needs three things: a power source, a load doing useful work, and conductors connecting them. Remove any one and you don't have a circuit. A battery without wires is just a paperweight with potential.