Two blonde twins were roofing a house. One was wearing a carpenters apron with pockets for nails. Most often, he would pull out a nail and then hammer it into the roof. But also, frequently he would pull out a nail, look at it, then throw it over his shoulder where it would go over the edge and fall to the ground.
One day, the second blonde, his twin sister, noticed what her twin brother was doing. After she had watched him a while she then asked, “Why do you keep throwing away almost every other nail?”
Her brother shrugged as he replied, “Because I can’t use them – their point is on the wrong end!…” The second blonde then said, “You dipshit airhead, those nails are for the other side of the roof!”