http://www.codinghorror.com/blog/archives/001203.html

"Let's say, hypothetically speaking, you met someone who told you they had two children, and one of them is a girl. What are the odds that person has a boy and a girl?"

I don't quite understand how there can be 2500 comments (he made another post about answering the question and came up with the wrong result) arguing about this when the answer is presented in the question:

"Let's say, hypothetically speaking, you met someone who told you they had two children, and one of them is a girl. What are the odds that person has a boy and a girl?"

So only one of two is a girl. So the other must be a boy then. Though what if the boy had a sex change operation? I can't be bothered to think about the odds of that. Another possibility could be that there was some kind of mutation that resulted in neither I suppose.