When a plug is inserted into the receptacle, both springs are compressed and the shutters then open, allowing for the metal prongs to make contact to create an electrical circuit. Because both springs must be compressed at the same time, the shutters do not open when a child attempts to insert an object into only one contact opening, and there is no contact with electricity. Many feel that tamper- resistant receptacles are an important next step to making the home a safer place for children.
If you do have TR receptacles you will notice that it can be difficult to insert the blades of a plug into the receptacle at first. As that receptacle is used more it will become easier to insert plugs as the TR device loosens with use.
I recall my mother telling the story about when I was around 5 years old and I stuck a car key into an outlet. She said I only did it once! Oh, and my baby crib was painted with lead based paint too!