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“In 1905, Britain's greatest puzzlist, Henry Ernest Dudeney, demonstrated his most famous creation at a meeting of the Royal Society in London. The Haberdasher's Puzzle is an equilateral triangle that is cut into four pieces that can be rearranged into a square. Sounds ordinary enough. Yet Dudeney's genius was that he connected the pieces together with hinges to form a chain - such that folding one way gets you a triangle and folding the other gets you the square. The "hinging" property of the Haberdasher's Puzzle, which Dudeney had made out of mahogany and bronze, has fascinated and delighted mathematicians for more than a century.” source...