Give following instructions: Cut a strip off a piece of paper and give it one twist. Glue the ends together. Then cut the strip in half. Before participants cut the strip in half, ask for the expected end result. The result is surprising for most: They don't get two strips, but one larger strip. Learning outcome: Results can be different from what was expected.
Explanation: This is a Möbius strip, a surface with one side and one boundary. The strip only has one edge that is twice as long as the strip, and cutting creates a second edge, half of which was on each side of the scissors.