Puzzles
Add one metre to a rope wrapped around the Earth's equator — how big is the gap?
A rope is tied snugly around the Earth's equator, roughly 40,000 km long. You splice in one extra metre of rope and pull it into a perfect circle, evenly raised above the ground all the way round. Is the gap between rope and ground big enough for a cat to crawl under?
Reveal the answer
Yes, easily — the gap is about 16 centimetres all the way round, plenty of room for a cat. Circumference equals 2π × radius, so adding 1 metre to the circumference only ever adds 1÷(2π), about 0.159 metres, to the radius — true whether you're wrapping the Earth or a tennis ball, since the object's actual size cancels out of the maths entirely. It's one of recreational mathematics' most reliable ways to catch people over-estimating how circumference scales with radius.