So we can either get a boy and a girl, but not both, because that is a single process. So we either get one or we get the other. So every time we have a child, usually we only produce one child, either a boy or a girl. So that's what we mean by mutually exclusive events. So let's look at example number three to demonstrate the sum rule. So if a couple has children, what is the probability that one of them is a girl and the other one is a boy?