[kgwgk]

> What they don't understand is probability.

Exactly. As the first reply to the first comment explains “The problem is that we don't know p(you're told at least one is a girl | they aren't both girls).”

It’s funny that the same commenter who writes that “to get the right answer you must be careful about conditional probabilities” finds that doing so is “splitting hairs about something boring and irritating.”

[JeffJor]

And when you don't know, you have to assume it is 50:50. Otherwise you get different answers for different-but-equivalent information.