Cleaning Elevators

A new hotel employee was asked to clean the elevators and report back to the supervisor when the task was completed. When the employee failed to appear at the end of the day the supervisor assumed that like many others he had simply not liked the job and left. However, after four days the supervisor bumped into the new employee. He was cleaning in one of the elevators. "You surely haven't been cleaning these elevators for four days, have you?" asked the supervisor, accusingly. "Yes sir," said the employee, "This is a big job and I've not finished yet - do you realise there are over forty of them, two on each floor, and sometimes they are not even there...."