Two blondes were roofing a house. One would pull out a nail and then hammer it into the roof. Then he would pull out another nail, look at it, then throw it over his shoulder. Blonde two eventually saw what blonde one was doing, watched him a while and then said, Why do you keep throwing out every other nail?.

The first blonde replied, Because their point is on the wrong end.

The second blonde then said, You airhead, those nails are for the other side of the roof!