An electric circuit is a path around which electricity can travel. There must be a source of electricity, and an unbroken path from one side of the source to the other. In our example below, the source is a battery.
The electricity travels from one pole of the battery, through the light and the switch (which is connected, or 'closed'), and back to the other pole of the battery. This path forms a complete circuit.

This simple circuit has just one device, not counting the switch ... it's the light bulb. If you were to open the switch, the circuit would be broken, and the light bulb would go out.

There are more complicated circuits, of course; let's look at one with two light bulbs on it.

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