What was he thinking?

OK.  So I'm going over some family images and as I was looking through them this one image made me about pee my pants I was laughing so hard.  I had forgotten this day.  The kids were SO dirty from summer playing and they were hanging upside down on my neighbors stair rail in her yard.  We have a few steps going up the hill to each house hence the rails.  Anyhoo, I guess I got my little one at just the right moment.  I just wish I knew what he was thinking to make a face like this...

What do you think he is thinking?  Write it in the comments.  Here's mine:  

"I think I have to poop."