These two men are building a house. One of them is
putting on the siding. He picks up a nail, hammers it in. Picks
up another nail, throws it away. Picks up a nail, hammers it in.
Picks up another, throws it away. This goes on for a while, and
finally his friend comes over and asks him why he is throwing
half of the nails away.

He replies, "Those ones were pointed on the wrong end." The buddy
gets exasperated and says "You idiot, those nails are for the other
side of the house!"