Calling all expert mother advice........

Maddy came home yesterday with her new library book she checked out. As I was making dinner she was finishing up her homework and she wanted me to read her book with her. I told her to sit on the counter and read me her book while I finished dinner. So I was kinda half listening and not really paying attention to what she was reading and then I heard her read, "Tiffany was the first girl in school to wear a bra, even though she didn't need one." My ears perked up a little and I thought what the heck is this book about? I looked at the book and it was a book about a girl named Amber Brown and we had read books about her before. Granted they were not chapter books as big as this book she was reading. So she continued, and again I started tuning it out a little until I heard, "We must......we must.....we must improve our bust. We better.......we better...... before we wear our sweater." WHAT?! She is seven! So I promptly said, "TIME TO EAT!" and made her put the book down and set the table. Now the thing is she had NO idea what that "chant" was all about. It was actually really really funny because I don't think there are many people out there that haven't heard the, we must improve our bust thing, but I don't think ANYONE has ever heard it said like Maddy was saying it. As she was setting the table she was dancing around singing it in the most melodic melody and it was making me laugh! She just had NO idea what it was, she thought it was a song or a cheer and she has no idea what "a bust" is. While this isn't TERRIBLE, I don't think she is old enough to be reading about these subjects yet. But maybe I am wrong? I don't know. I don't know if I should not make a big deal out ot it and just read it together with her and see what questions crop up from it. Or if I should conveniently misplace it for her until her next library day. Or if I should talk to her out right about it and explain things but then that opens a whole other can of worms explaining what a "bust" is and why it has anything to do with wearing a sweater. But I DO NOT want her teachign "the song" to her friends at school! It is a fourth grade level reading book (obviously aimed at an older audience) but Maddy reads well enough to read it. So anyway what do I do?! All you moms of girls out there PLEASE help me with this one. She has already put the book on the kitchen counter this morning and informed me that we are reading it together when we get home. I think most of it is going WAY over her head but WHAT DO I DO? And by the way Liz I can't promise that Sam won't go home from Thanksgiving singing the "we must improve our bust" song. ;)