Block and Bolt Riddle

[michaelgcros]

A teacher of mine once had a large block of wood out of which had been cut a 4 inch notch. On one side of the notch a hole was drilled. Through that hole passed a long bolt, the top of which was on the inside of the notch. The bolt was continuous and could be moved back and forth inside the hole, but it could not be removed as it was longer than the notch itself.
We were told that this was not a trick, but was explainable by logic alone. I have no reason to think he was lying, but have yet to figure out the solution. Please help. Thanks.