A nice mathematical puzzle, with a solution anyone can understand, is to determine the direction a bicycle went when you come upon its tracks. The answer involves thinking about tangent lines, geometric constraints and the bicycle’s steering mechanism. Once you learn the trick, you’ll find yourself using it every time you happen upon a bike trail.

The question goes back to the early days of the bicycle age and a 1903 Sherlock Holmes story by Sir Arthur Conan Doyle called “The Adventure of the Priory School.” Surprisingly, Holmes did not analyze the tangents as we do in the video and his reasoning in the story was incorrect. A geometric approach to the question was included in a 1990s “Geometry and the Imagination” course that John Conway, Peter Doyle, Jane Gilman and William Thurston taught at Princeton and the Geometry Center at the University of Minnesota. Holmes’ error is discussed in the book “Which Way Did the Bicycle Go?” by Joseph D. E. Konhauser, Dan Velleman and Stan Wagon.

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More videos from the Mathematical Impressions series.