Three Part Job

A passerby noticed a couple of city workers working along the city sidewalks. The man was quite impressed with their hard work, but he couldn’t understand what they were doing.
Finally, he approached the workers and asked, “I appreciate how hard you’re both working, but what the heck are you doing? It seems that one of you digs a hole, and then the other guy immediately fills it back up again.
One of the city workers explained, “The third guy who plants the trees is off sick today.”