Martin Gardner's paradox of the second ace
A bridge player is dealt 13 cards and announces, 'I have an ace.' What's the probability she has at least one more ace? Now suppose instead she'd said, 'I have the ace of spades.' Does naming that specific card change the odds of a second ace, and if so, which way?
Reveal the answer
Naming the specific ace raises the odds. Say only 'an ace' and the chance of a second ace is 5359/14498, about 37 percent. Say 'the ace of spades' specifically and it jumps to 11686/20825, over 50 percent. The paradox traces back to mathematician J.H.C. Whitehead in the 1930s, but Martin Gardner made it famous in his April 1957 Scientific American 'Mathematical Games' column.