I seriously wonder how he came up with this idea.

i think the same sound is made from each butt. i am not hearing a difference. =(

Because not hollow.  (Cannot believe the XY is unhappy about the SOUND.)

i just feel sorry for the two butts in the middle!  they are definitely his favorites!

Shouldn't you feel happy for them because they are his favorites?

i would like to see the face while this is going on. Are they laughing? Are they miserable? Are they quiet professionals? 

i am picturing them as quiet professionals.  at the end of the video, when he's rubbing lotion on them and apologizing, they do start to giggle...