Solution #63: Bookworm
Only 1/4 inch -- the thickness of the two covers.
Put a paperclip on the first page of one book, and another on the last page
of a second book. Put both books on a shelf together, the first on the left
and the second on the right,
and you will see why this surprising answer is correct.
Source: Fixx, James F
Categories: Reasoning, Pitfall, Fractions