When the giant town-hall clock chimed 2 o’clock, an ant resting by the number 2 woke from its nap. Spotting that the minute hand was edging towards it, the ant started walking clockwise round the rim ...

Three short-sighted spiders are clustered at the vertex of a wire frame in the shape of a tetrahedron. The spiders know that there is an ant walking around the frame, but they have no idea where it is ...