An intelligence test

A father had two sons and he wanted to be sure, when he died, to leave all his money to the more intelligent of the two. So he gave them a test.
"Look",he said, "here are three ribbons -two white and one black. I'm going to pin one ribbon on each of your backs, and you won't know what colour it is. There you are. Each of you can see the other on's ribbon, but you can't see your own. Now, go into the other room. The fisrt one who comes and tells me what colour ribbon he's wearing will be my heir and inherit all i possess."The two boys went out.
After a minute or two, the younger son came back and correctly told his father that he was wearing a white ribbon."Good boy,"said the father."How did you work it out?"
What was the boy's answer?