Walls & Lines
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A popular classic puzzle that involves a large square divided into 5 "rooms". The object of the puzzle is to cross each "wall" of the diagram with a continuous line only once.
(note the uncrossed wall - marked with circle)
[edit] Solution
The proof is as follows: "A continuous line that enters and leaves one of the rectangular rooms must of necessity cross two walls. Since the three bigger rooms have each an odd number of walls to be crossed, it follows that an end of a line must be inside each if all the 16 walls are crossed. But a continuous line has only two ends, so the puzzle is unsolvable."
The above proof is provided by Martin Gardner.
