Trihexaflexagon
Fold a strip of paper and glue the ends together to create a flexagon that displays different images as you turn it "inside out." This, the simplest of the flexagons, is called a trihexaflexagon because it displays three different images in a hexagon. There are a variety of flexagons with different shapes and number of images.
Steps
 Print the pattern doublesided and cut out the pattern piece.
 Follow the folding directions printed on the strip: first make a mountain fold where marked, then a valley fold. In the third step, "tuck under," switch positions of the two overlapping triangles; i.e. place the triangle labeled "tuck under" below the other triangle. Finally, fold the protruding triangle over and glue the two faces labeled "glue here" together.
 To use the flexagon, pinch two adjacent triangular faces together and push the opposite edge in.
 With the flexagon pinched together, you can open the flexagon to reveal a new image. If you cannot open the flexagon at this point, repeat the previous step, but pinch two different triangular faces together.
 Repeat steps 3 and 4 until all images are revealed.
Notes

Flexagons were discovered by Arthur H. Stone in 1939 when he was a 23 year old graduate student at Princeton University. He trimmed his Americansized paper to fit in his Britishsized notebook. He folded the resulting strips of paper and discovered the trihexaflexagon.
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