Are You as Smart as a Snake? Puzzle Solution Revealed

Earlier today, a puzzle was presented to readers involving two snakes in a cage, one long and one short. The solution is now provided.

Two snakes of equal width, one long and one short, are confined in a cage.

The challenge was to design two escape passages, A and B, leading from the bottom of the cage, such that the short snake can escape through A but the long snake cannot, and the long snake can escape through B but the short snake cannot.

The passages must have no moving parts, trapdoors, or levers. The snakes have circular cross-sections with constant diameter along their bodies. They can wriggle but cannot squeeze through passages narrower than their width.

Passage A requires a loop longer than the short snake but shorter than the long one. A sketch illustrates an effective escape route.

The passage is a tube with the same diameter as the snake. The snakes follow the arrows. When the long snake doubles back and reaches the point where the tube meets itself, it cannot proceed because its own body blocks the way. The short snake experiences no such problem and can pass.

Note that a snake cannot turn right at the junction because the gap between the top and bottom tubes is insufficient. A cross-sectional diagram shows the outline of two overlapping circles. When the snake occupies the top tube, it cannot squeeze into the bottom tube, so it must follow the arrows into the loop. Upon completing the loop and encountering itself, the long snake’s body in the top tube obstructs part of the lower tube, preventing passage.

Passage B is simpler: the route passes through a hole in the floor. The short snake cannot cross without falling in, while the long snake, due to its length, does not have this problem, assuming non-zero rigidity.

“Thanks for all the herpetological jokes below the line in the original post. Not all of them were as painful as a snake bite!” the author wrote.

The author has been setting a puzzle on alternate Mondays since 2015 and is always seeking great puzzles. Those with suggestions are invited to email.

This article originally appeared in The Guardian.

The solution demonstrates a clever application of geometry and snake physiology.