Laurence: I'm glad we have 3 boys. I don't think we need any more babies in this family.
Me: Well, that's a good thing, because we won't be having any more babies.
Laurence: If you did have another baby, it would be a girl.
Me: Oh? That's what you thought with Alastair, too, but he's a boy.
Laurence: But the 4th baby is always a girl.
Me: Really? How do you know that?
Laurence: The plumber.
Me: What? What plumber?
Laurence: Remember that plumber that was here who had six kids? He told you you should have another baby. (For the record, it was a Heating & Cooling Technician, and it was last summer that he was here...)
Me: Oh, yes, I do remember him telling me that, but---
Laurence: AND he said, "The 4th one's always a charm!" So the 4th baby is always a girl. Girls are charms, not boys.
So there you have it.