A Strip of Paper Can Have Only One Side

Cut a long strip of paper. Twist one end half a turn. Tape the ends together. You now have a Möbius strip, named after the German mathematician who studied it in 1858.

Draw a line down the middle of the strip without lifting your pen. The line goes all the way around, comes back, and meets itself. You never crossed an edge. You never lifted the pen. The strip only has one side.

Cut the strip down the middle along your line. It does not split in two. It becomes a single longer loop with a full twist in it. Cut that one in half again and you get something even stranger: two interlinked loops that cannot be pulled apart without scissors.

Möbius strips show up in conveyor belts, where the one-sided wear extends the lifespan. They are also the shape of the universal recycling symbol, which has been pointing at itself for sixty years and almost nobody notices.

If your child enjoys this one, the next step is the penny bridge experiment, where the same paper, folded a different way, holds twelve times its own weight - or the Math Facts That'll Make You Say "Wait, What?" Series