saw this pop up the other day.
You have a bicycle, and your friend is holding it up so that it will not fall over, but he is not applying any pressure to it in any direction. You rotate the crank so that one of the crank arms is facing straight down, and then you tie a length of string around that crank arm. The setup will look something like this:
With your friend holding the bike up, you pull back on the string. Will the bike move forward, backward, or stay in the same spot?
Hint: The bike's rear wheel will not skid.