This is based on a puzzle I heard from @colinthemathmo, who wrote it up here; he heard it from @DavidB52s, and there the trail goes cold.
The Mathematical Ninja lay awake, toes itching. This generally meant that a mission was in the offing. Awake or dreaming? Unclear. But the thought had implanted in the Ninja brain: pull a rope taut around the equator, add six metres to one end and tie it to the other. Now lift the rope upwards until it’s taut. How high do you have to go?
Eyes closed. The rope will follow the equator most of the way around, then take off in a straight line. Let’s call…
Continue reading here:
The Maths Echo Chamber 