Squaring the Circle
A marble runs round a square.
Not really a puzzle, but how it works may be.
The ancients proposed a challenge  to construct a square with the same area as a given circle using only a straight edge and pair of compasses.
Some 4000 years later, in 1882 it was at last proved to be impossible "because of the transcendental nature of pi..."
Since, "Squaring the Circle" has come to mean, "Attempting the impossible".
