Two carpenters are nailing up siding one day. The first carpenter grabs a nail from his pouch, examines it, and then tosses it in the trash. He proceeds to grab another nail, examine it, and then hammers it into the siding. He repeats this process several time. Finally the second carpenter turns to him and asks “Why are you doing that?” to which the first carpenter replies “Half of these nails have the head on the wrong end!” The second carpenter says back to him “You idiot! Those are for the other side of the house!”