Okay this roofer takes a handful of nails out of his pouch, throws some away, hammers a few, throws some away, hammers a few more, etc.

The foreman watches him for a while, then says, "Hey! What the heck are you doing? You're throwing half the nails away!"

The guy says, "The heads are on the wrong end."

The foreman slaps his forehead:

"You idiot! Those are for the other side of the roof!"