Bolt & Block Riddle

[michaelgcros]

A teacher of mine once had a large (10" x 10" x 10") cubical block of wood with a notch -- four inches long and spanning its width -- cut into the top it. A hole had been drilled from the inside of the notch to the outside of the block. Through that hole ran an eight inch bolt. The top of the bolt was on the inside of the notch. It therefore could not be removed. It could, however, be moved back and forth within the hole.
I was told that this was not a trick -- that the bolt and the wood were true -- and that it could be explained logically. But to this day I have been unable to do so. Please help.