Left image: A vacuum cleaning machine that goes around the floor on its own and cleans the floor. Everything  around it looks squeaky clean, and is shining.
Right image: Close up of a vacuum cleaning machine that goes around the floor on its own and cleans the floor. Everything  around it looks squeaky clean, and is shining, but it has a lot of dust on it except one line on it that looks clean. A persons fingertip is visible, and it is covered in dust.
This is funny, because the machine while doing its job cleans everything but ends up being dirty itself.