A triangle that looks perfectly solid and cannot possibly exist
Look at a drawing of a triangle made from three beams that each look straight and connect at right angles — yet the whole shape loops back on itself in a way no real solid object could. Why does this look so convincing, and can it actually be built?
Reveal the answer
It can't exist as a rigid solid in ordinary three-dimensional space — each corner is locally consistent, but the drawing cheats perspective globally, so the three corners can never agree on a single 3D shape. Swedish artist Oscar Reutersvärd drew the first version in 1934; psychiatrist Lionel Penrose and his son, mathematician Roger Penrose, independently rediscovered it and published it in 1958, calling it 'impossibility in its purest form.' M.C. Escher used it soon after in his 1961 lithograph Waterfall.