What's That Smell?

This morning, Caleb got up early and requested to get breakfast at McDonald's before catching the bus for school.  We had not done that in a while, so off we went.  We ate inside, and then hurried back to the car.  As we were both getting in the car, I smelled it.  Caleb suddenly asked, "What's that smell?"  Simultaneously I said, "Ewww.  Did you poot?"   He said, "Yea, I did.  But I didn't think it would be THAT bad!"