My 5-year-old nephew registered for kindergarten last week, and during his pre-test, my brother-in-law asked my sister why Caleb's shoes were off. Kim didn't know, so when Caleb came back over to them, they asked him.
He said, "Those shoes were hurting my feet. I couldn't think with them on."
So apparently, while some require a thinking cap on their heads, Caleb requires those non-thinking shoes off his feet. ;-)