This is a classic puzzle, many of you have probably done it at some point in time. I was fortunate to have been introduced to this puzzle by none other than Prof. Edgar W. Dijkstra, who taught me a course on basic logic in University of Texas at Austin, and the question was given to me as my final exam in a 1:1 setting. Am so proud of that fact, that I am sharing a photograph from that day (at the end of the post), I am hoping that those of you who are from Computer Science background will appreciate why I feel so proud!

The question in its current form is taken from one of the Martin Gardner books, Mathematical Puzzles and Diversions, Chapter 3, puzzle #4.

A logician vacationing in the South Seas finds himself on an island inhabited by the two proverbial tribes of liars and truth-tellers. Members of one tribe always tell the truth; members of the other always lie.

He comes to a fork in a road and has to ask a native bystander which branch he should take to reach a village. He has no way of telling whether the native is a truth-teller or liar. The logician thinks a moment and then asks one question only. From the reply, he knows which road to take.

What question does he ask?

As always, please send your answers directly to me at alokgoyal_2001@yahoo.com. If you like the puzzle, please share it with others. If you have interesting puzzles to share, please send them to me at my e-mail given above.

Happy holidaying!

Photo with Dijkstra (10th Feb, 1994)