Two men were working for the city public works department. One would dig a hole and the other would follow behind him and fill the hole in. They worked up one side of the street, then down the other, then moved on to the next street, working furiously all day without rest, one man digging a hole, the other filling it in again.
An onlooker was amazed at their hard work, but couldn't understand what they were doing. So he asked the hole digger, "I'm impressed by the effort you two are putting in to your work, but I don't get it - why do you dig a hole, only to have your partner follow behind and fill it up again?"
The hole digger wiped his brow and sighed, "Well, I suppose it probably looks odd because we're normally a three-person team. But today the lad who plants the trees called in sick.'"