MATH PUZZLE: Can you replace...

A young mother finds out she is pregnant again, and she tells the good news to anyone who will listen. One day when the woman and the boy are out shopping, a friend of the mother asks the little boy if he was excited about the new baby.

"Oh, yes!" the little tyke says. And having overheard some of his parents' private conversations, he adds, "And I know what we are going to name it, too. If it's a girl we're going to call it Mary, and if it's another boy we're going to call it Quits."