Harvey Kneeslapper-B in your bonnet

[Rubber Duckie]

(Harvey come sin giggling. About the same time, Fat Blue shows up)
Harvey: Hey, do you have a B in your bonnet? (Laughs widly)
Fat Blue: What?
Harvey: I say, do you have a B in your bonnet? (Laughs wildly)
Fat Blue: No. (Harvey promptly slaps a letter B on to Fat Blue's bonnet)
Harvey: Now you do. (Laughs hysterically;end music plays)
Fat Blue: AAAAAAAAAAARRRRRRRRRRRRRRRGGGGGGGGGGGGGGHHHHHHHH!

 

[minor muppetz]

I don't get it.