Brainteaser-Type Questions

Sometimes, interviewers ask "brain teaser" type questions to see if you are creative. I personally don't think it helps to memorize these questions because there's too many of them to memorize. But, if you happen to have heard a few of them before an interview, it doesn't hurt, either. Here's the list I have so far:

How do you swap two real-numbered variables without using a third temporary variable?
What is the fastest way to figure out what number is missing from a list of numbers from 1 to 100?
Why are manholes circular?
There are three light bulbs in a room, and outside the room there are three switches that control the lights, but you do not know which switch is connected to which light. You can do anything you want to the switches before entering the room, and then, upon observing the lights in the room afterwards, you have to determine which switch goes with which light. How do you do it?