We can either get a girl and a girl, which is also not what we want, but the other two outcomes are boy and a girl and girl and a boy. And in both of these cases, we basically get one girl and one boy. Now, these two events are mutually exclusive. And what that means is if this event took place, this event cannot possibly take place. And likewise, if this event took place, this event could not have possibly taken place. And so because they're mutually exclusive, we apply the sum rule and we have one fourth plus one fourth.