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A fascinating 4-piece puzzle with 2 identical pieces and 2 identical mirror images which interlocks into a tetrahedron. This puzzle was invented around 1988 by Leonard Gordon. Subsequently rediscovered by Hirokazu Iwasawa and also myself. Bernhard Wiezorke called it "Stan's Tetrahedron" because he thought it had been invented by Stan Isaacs (Stan did invent a different puzzle with the same final shape).
This is one of the few puzzles here that can be assembled using rigid pieces. The final assembly is very stable. Made from 12mm diameter hollow spheres. Update Feb 14 2010: the new version has no holes visible when assembled.