Most electrical appliances are connected to the mains using three core cable.
The insulation covering each wire is colour coded for easy identification:
live wire – brown
neutral wire – blue
earth wire – green and yellow stripes.
The live wire carries the alternating potential difference from the supply.
The neutral wire completes the circuit.
The earth wire is a safety wire to stop the appliance becoming live.
The potential difference between the live wire and earth (0 V) is about 230 V.
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The neutral wire is at, or close to, earth potential (0 V).
The earth wire is at 0 V, it only carries a current if there is a fault.
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You should be able to explain:
that a live wire may be dangerous even when a switch in the mains circuit is open
the dangers of providing any connection between the live wire and earth.
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