Hexaflexagon
Dashed line: fold toward you (valley). Dash-dot line: fold away (mountain). The arrow shows which way the paper moves.
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1. Count three triangles from the left end. Fold the rest of the strip toward you along the line just after them. Press it flat.
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2. Count three more triangles from the first fold. Fold the rest of the strip toward you along the line just after them. Press it flat.
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3. Fold the triangle that sticks out behind the paper along its line, so it lies flat on the back of the first triangle. Glue or tape it there.
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Done. Now you have a flexagon: pinch two triangles together, push the opposite corner in, and open the middle to find the hidden face.
A traditional model. The three-face flexagon. Arthur Stone found it in 1939 by folding strips trimmed off American notebook paper at Princeton; since Martin Gardner wrote about it in 1956 it has been folded freely in classrooms everywhere.