Missing

A conversation between me and one of my students last Friday:

Him: Mrs. C., do you notice anything different about me?

Me: Do you have a new backpack?

Him: No.

Me: A new coat?

Him (now exasperated): NO! I'm missing part of my butt.

Me (extremely confused): What?!?!?!

Him: Last night at basketball practice, I ran my butt off and now I think part of it is missing.

Me: Oh, honey... that's just an expression that people use when they work really hard or play really hard. I really don't think that part of your backside is really missing.

Him: I don't know... It IS getting smaller...