Fringe, Season 1

Astrid:
It also gave him four extra nipples.

Peter:
Maybe he mutated into an opossum.

Walter:
No, opossum's have 15 mammaries, unlike most mammals, who have an even number of nipples. Hence, the one half rule regarding nipples.

Astrid:
The one half nipple rule?

Peter:
You just had to ask, didn't you?

Walter:
The typical number of young in the litter is usually equal to half the number nipples on the animal. The number of nipples being the maximum litter size. Humans, for example, one child is the typical. Maximum, twins. Barring extraordinary cases when the young exceed the number of nipples.

Peter:
God, I hope I never have to hear him say the word "nipple" again.