Why the bumps?

If you were to take apart an outlet and look at the contact(s) that the prongs slide into, you would find that they have bumps on them. These bumps fit into the holes so that the outlet can grip the plug's prongs more firmly. This prevents the plug from slipping out of the socket due to the weight of the plug and cord. It also improves the contact between the plug and the outlet. If one of the prongs or the wires inside them becomes dislodged, electricity won't flow as efficiently through the device. This misdirected current could, at times, ruin appliances or may even shock the user because it is no longer grounded.