Electricity

[Honus]

As I mentioned in the thread about CFLs and LEDs, electricity confuses me. Here are two questions I have have never been able to figure out.

First, I can't figure out why this is true:

Originally Posted by spdrun

...if the lamp isn't polarized and it's plugged in with hot connected to the socket shell, you can get a tickle when removing a bulb.

Second, what it the hell is alternating current? I understand that the voltage alternates, but what is physically happening inside the wires? Are the electrons vibrating back and forth?