Around the Block
Mark every vertex of a cube and every midpoint of its six faces. An ant sets out to visit all of these points, walking only along face diagonals, never retracing a step.
- The cube has 8 vertices and 6 face-center points — 14 stops in all.
- Every face diagonal connects a vertex to a face center, never vertex to vertex or center to center.
- The ant can't repeat a diagonal it's already walked.
Can the ant visit every vertex and every face center?
Reveal the answer
No. Face diagonals only ever connect a vertex to a face center, so any walk along them must alternate strictly between the two kinds of point. A cube has 8 vertices but only 6 face centers — an even alternation can never cover an unequal split, so the full tour is impossible.
From Arthur Engel's Problem-Solving Strategies (1998), as collected on Futility Closet.