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Take a strip of paper.
Give it a half twist and join the ends. You now have a Möbius strip - a piece of paper where the flat area is all one side.
Join the ends without a half twist and you just have a loop, with the flat area is two sides.
How can you turn a strip of paper into something with three flat sides? (No cutting, and with just one join, as in the previous two examples.)
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