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!"