Nail

These two Poles 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 are for the other side of the house!"