The Boo Boo Comparison

My second swim class this morning compared boo boo's.  It went like this:

B:  "Miss Margo, look at my boo boo."
Me: "How did you get that?"
B:   "I fell of my bike."
O:  "Miss Margo, I have a boo boo, too.  It's on my knee."
Me:  "Let me see.  Oh, that's a good one."
O:  "Do you have any boo boo's?"
Me:  "I have one on my knee, too."
B:  "How did you get it, Miss Margo?"
Me:  (mumbling) "Shaving . . ."