Möbius strip: the mindbending shape with only one side.
It's very simple to create: take a long piece of paper, give one end a 180-degree twist and then stick the ends together. The simple shape just created has only one side. What??
You can test this by running your finger over the surface and you'll cover the entire shape and end up back where you started.
Try to follow the red ball in the animation as it follows the surface over the entire loop.
Möbius strips have a couple of other interesting properties including how cutting them in half down the middle simply makes a longer loop, and cutting a third of the way in over the full length will reveal two distinct yet interconnected loops. Fun to try!
In case it's handy here's a static Möbius strip sketch