But what happened to your belly button?

Eirena climbed on my lap to read a book. She started to lift up my shirt. I asked her what she was doing. She said she wanted to look at the baby- (a sister, according to her). I told her she couldn't see the baby because it was inside. A couple seconds of pondering pass as she examines my distended belly. And then, looking and my very stretched out belly button, she asked with great concern, "but what happened to your belly button? where did it go?"