So there were two guys on a roof, pounding nails...

So there were two guys on a roof, pounding nails. One guy pounded a nail in, then picked up another. He was holding the nail upside down. He unexpectedly threw the nail away. He picked up another nail, right side up this time, and pounded this in. He eventually threw so many upside down nails away, that his friend came over. "Eh, what you doing? How come you're throwing away all those nails?" he asked. "Because they're upside down," the friend replied. The other guy looks at the friend, then, after some thought, says, "You Idiot, save them for the ceiling!"