Why is this a toothbrush?

Lily: (holding Daddy's toothbrush) why is this a toothbrush?
Daddy: because that's what the manufacturer wanted it to be.
Lily: but WHY is it a toothbrush!?
Daddy: why are you a little girl?
Lily: you mean, why am I a big girl?
Daddy: (giggling) yes. Why are you a big girl?
Lily: because I brush my teeth.