One afternoon, Bob and his boss were on a roof pounding nails. Bob would pound a nail in, then pick up another. He was holding the nail upside down. After looking at it, he suddenly tossed the nail away.
He picked up another nail, right side up this time, and pounded it into the roof. He eventually tossed so many upside down nails away, that his boss came over. "Bob, what are you doing? How come you're tossing away all these nails?" he asked.
"Well, they're upside down," Bob replied.
Shaking his head, his boss looks at Bob and yells, "You idiot, save them for the ceiling!"