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."