Two framers were nailing plywoods.
The first framer pulled nails from his pouch, sometimes nailing them and sometimes throwing them away.
The second framer asked: "hey, why are you throwing away half the nail?"
"I tell you, said the first framer, when I pull a nail out, if it points towards the house, I nail it. But if it points towards me, I throw it away."
"You idiot said the second framer. If it points towards you, it's supposed to be nailed on the other side of the plywood."