Birth order per se doesn't matter; probability mass does. Birth order is an easy way to show that you're twice as likely to get a boy and a girl as you are to

Birth order per se doesn't matter; probability mass does. Birth order is an easy way to show that you're twice as likely to get a boy and a girl as you are to get two boys. But you can ignore birth order and say instead "let k be a binomially distributed random variable with n=2 and p=1/2. If you know k>=1 what is p(k=2)?". Then the answer is a straight comparison of p(k=2) to p(k=1), where we note that '2 choose 1' is 2, and '2 choose 2' is 1. Note that ordering per se isn't entering into this, but to someone without probability background, it's a bit obtuse. Describing the probability space in terms of birth order is not strictly necessary but helps a lot of people grasp the concept.