This one was on University Challenge, so I won't claim credit:
Lay a rope all the way round the Earth so that the two ends are just touching and there is no slack in the rope.
Now, raise the rope one metre off the ground all the way around the Earth.
How big will the gap between the two ends of the rope be?
Put your hands up and no shouting out!
If you have an answer then post it so that the text is formatted to the same colour as the background and can only be read by highlighting, like this:
So it won't spoil it for others.
Enjoy.
