In this article, the author discusses two probability puzzles that often confuse people. The first puzzle involves determining the probability of a person having two boys given that they have at least one boy. Many people wrongly assume the probability is 1/2, but the correct answer is actually 1/3. The author then introduces a variant of the puzzle where the person specifies their child is a boy born on a Tuesday. This twist makes a surprisingly big difference, and the correct answer is 13/27. The author emphasizes the importance of setting up the problem correctly and points out that representing real-world problems accurately can be challenging when calculating probabilities.

https://www.maa.org/external_archive/devlin/devlin_04_10.html