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@Arducius
Drakien has the right logic behind this one. I instinctively thought the answer would be 1 in 2, because you know the first child is a boy, it has no bearing on the sex of the second child and the chances of a child being a boy or being a girl are both (for the sake of simplicity) 1 in 2. But there is a flaw in the logic here because we are assuming that the one child we know the sex of has been assumed to be the first (or second, it doesn't matter).
Therefore we have to look at all possible combinations (BB, BG, GB, GG) and see how likely it is that the desired outcome would occur. We can discount GG because we know at least one is a boy. Leaving 3 possible outcomes, with only 1 being the desired result, therefore 1 in 3.