Why do some Plugs spark when disconnected?

The electricity supplied to a building travels round a circuit in cables under the floor and back to the main supply point. A Plug socket is a point where there is a deliberate break or gap in that circuit. When you plug something in, the circuit is complete, and the electric current passes through the apparatus, making it work.
If you pull the plug out when the current is switched on, the contact surface of the plug pins gets smaller as it is pulled out. The ends of the pins heat up very quickly and then the remaining electricity in them jumps across the gap between the pins, which we see as a spark. You should always switch off a plug before disconnecting it.