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!"