One triangle, four pieces, hinged into a square
Cut an equilateral triangle into as few pieces as possible so they can be rearranged into a perfect square. Henry Dudeney's solution goes further than the challenge asks — his four pieces are hinged together in a chain, so swinging them one way forms the triangle and swinging them the other forms the square.
Reveal the answer
Four pieces. Dudeney presented the hinged version to the Royal Society in 1905; it's considered one of the most elegant dissections in recreational mathematics, since the same physical chain of pieces can fold into two completely different regular shapes.
— Henry Dudeney, The Canterbury Puzzles — 1907; puzzle first presented 1905
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