That Man's Father Is My Father's Son

As you're passing a park bench just outside the bounds of the party, a man sitting on it starts talking to you. You're fairly sure he's a vagrant hitting you up for some change, but against your initial impulse you stop anyway. You're intrigued by his first statement.

"I've got a challenge for ya. I bet ya I can tell you how many children your father had, just by looking in your eyes. Whaddya say?"

"Just by looking in my eyes?"

"Yeah, that's all it takes." The man lays out a 10-chip piece on the park bench, and you set your own beside it. He glances at your eyes for just a second and then scoops up the chips. "Your daddy ain't never had no kids. Your momma had all the kids."

How do you react?