MATH PUZZLE: Can you replace...

A baby was born with the ability to talk.
The first thing he said when he was born was, "Are you my mom?"
"Why, yes!" his mother said. "I am!"
"Well," the baby said, "I wanted to thank you for taking such good care of me before I was born." Then he looks around the room and says, "Are you my doctor?"
"Yes, I am!" says the doctor.
"Well, I just wanted to thank you," says the baby, "for taking such good care of me during the delivery."
"You're very welcome," says the doctor.
The baby looks around the room and says, "Hey, are you my father?"
Overcome with pride, his dad says, "Yes, I am!"
The baby says, "Come here for a minute. I want to show you something. Bend down." The father complies, and the baby starts poking him in the forehead. "How does that feel?! Hurts, doesn't it?"