Two Irishmen were working for the city public works department.

Paddy would dig a hole and Mick 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, Paddy digging a hole, and Mick filling it in again.
An onlooker was amazed at their hard work, but couldn't understand what they were doing.
So he asked Paddy, “I'm impressed by the effort you two are putting into 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?“
Paddy wiped his brow and sighed, “Well, we're normally a three-person team, but today the lad who plants the trees called in sick”.