Why three prongs?
In college I lived in an old apartment building that had been updated to have 3-prong outlets. One day I was wiring up some audio equipment for a party. As I hooked my computer to a a receiver in another room the audio cable immediately started sparking and the rubber insulation melted off. I yanked it out of my computer. Luckily the only real damage done was the sound card for my computer and the destroyed cable.Later I did some investigation. First I bought one of those standard outlet testers. It seemed to suggest that everything was fine with the outlet. Then I did some more investigation. I got a volt-meter and started measuring the voltage from the different prongs. Finally I measured the voltage compared to a copper pipe in the house.Turns out that both the neutral and ground wires were actually hot (as measured against the grounded pipe), and the "hot" wire was the ground. The outlet in the other room had been wired correctly. The shielding on the cable was grounded so suddenly it sent 120 volts down the audio cable when I plugged it in—destroying it in seconds.The electrician didn't believe me when he came in to fix it, but was finally convinced when he checked against a pipe.Just goes to prove that you can't make any system fully fool-proof.