If a woman has two children and one is a girl, the chance that the other child is also female has to be 50-50, right? But it’s not... The possibilities are girl-girl, girl-boy and boy-girl. So the chance that both children are girls is 33 percent. Once we are told that one child is female, this extra information constrains the odds. (Even weirder, and I’m still not sure I believe this, the author demonstrates that the odds change again if we’re told that one of the girls is named Florida.)
From George Johnson's review of Leonard Mlodinow's The Drunkard’s Walk: How Randomness Rules Our Lives, in this Sunday's New York Times Book Review.
The fact that the Book Review editors chose the first sentence of the above excerpt as a call-out sentence (in the paper edition) is especially alarming.