Interactive recreation of the Haberdasher's problem
This equilateral triangle has been carefully split into four pieces. Drag, drop and rotate the pieces to turn the triangle into a perfect square.
This puzzle was first solved by Henry Dudeney in 1903, and is named the Haberdasher's puzzle because the four shapes bear a resemblance to pieces of cloth you might find in a haberdashery (a shop selling bits and pieces related to sewing). This type of puzzle is called a dissection puzzle.
Anyone finding the above puzzle interesting, may also be interested in a number of books by Professor Greg N. Frederickson dedicated to similar geometric puzzles.