Thanks for the funn! I love brain teasers.
Here's my favorite:
A door leads to a completely sealed room from which no light or noise can escape once the door is closed. The room contains a bare lightbulb which is turned on and off using a lightswitch outside the room. Next to the working light switch are two identical dummy switches that do nothing.
Now for the puzzle. The door is closed and you are standing outside the sealed room next to the 3 switches. You can flip the 3 switches as many times as you want and leave them in any up/down configuration you want. However, once you open the door and enter the room you cannot touch the switches. How can you tell which is the correct switch to turn on the light bulb after you've entered the room?
And no, you cannot pry off the switch covers to see which is wired