Move six coins one at a time so every coin always touches two others
Arrange six pennies in a triangle: three along the bottom, two above them, one on top, each coin touching its neighbours. Now move the coins one at a time into a straight line of six — but after every single move, the coin you just moved must be touching exactly two other coins. How is it done?
Reveal the answer
The final coin placed has to end up touching two others, which is impossible if it lands on either end of a flat line — so the trick is to stack the last coin on top of two others in the row rather than trying to keep everything in one plane throughout. Henry Dudeney set the puzzle in his collection '536 Puzzles and Curious Problems'.