Stumped

Nicky just stumped me with a clever question. He was complaining that I was putting him to bed instead of Anna, who was settling Kitty. The conversation went like this:

Nicky: "Why can't you put Kitty to bed?"
Me: "Because I can't feed her"
Nicky: "Why do you have nipples then?"
Me: "Er..."

I know, but being put on the spot like that caught me off guard! I'm sure it's the first of many similar questions, I'd better brush up!