Foot in hand

My son was just sitting next to me, scratching his head with his foot, but holding his foot in his hand.
Me: Why are you scratching your head with your foot?
Son: Because it itches.
Me: But why are you using your foot?
Son: Because my head itches!
Me: Why your foot and not your hand?
Son: I can't use my hand.
Me: Why not?
Son: Because my foot is in it and it's full!