A bird perches on a bare high-power line, and nothing happens to the bird. A man standing oa the ground touches the same line and gets a fatal shock. Why ? 

Ans: No current passes through the bird because the circuit is not completed and its body is at the same voltage as the live wire. 

When a man touches the same line the circuit is completed. There is a large potential difference between his hand and feet. A large current flows through his body and he gets a fatal shock. [But if the man wears shoes with insulating soles he will not get a shock.]