Addison learns the B word

We were in the car, when Addison piped up from the backseat.  Here's how the conversation went.
A: Mommy, is witch a bad word?
Me:  No
A: O.K.!  Witch! Witch! Witch! Witch!
In the course of her saying that, it sounded like she said the b word.
Me:  Wait, wait. Saying witch is o.k. but if you say witch with a b sound at the beginning, that's a bad word.  So, wu, wu, w with witch is o.k.  But, bu, bu, b with witch is a bad word.  So, the word with a w is o.k. but the word with a b is a bad word.
Things were quiet for a few seconds, then in a really sad voice, Addison asked, "Do you mean I can't say butterfly?"