Title text: Science joke. You should probably move along.
A Cueball-like guy asks Cueball, the store manager, how they keep the store so clean, and he is told that they have hired Kepler, a hard worker who doesn't mind the monotony and sweeps out the same area every night.
Johannes Kepler was a German mathematician, astronomer, and astrologer, best known for his laws of planetary motion. By using Tycho Brahe's observations of our solar system (Brahe gave Kepler the job of observing and explaining the motion of the planet Mars), Kepler was able to deduce that planets in the system do not move in a circular orbit around the Sun, but rather in an elliptical one. In doing so, he directly contradicted Brahe's own conviction that the Earth was the centre of the universe.
According to Kepler's Second Law, "A line joining a planet and the Sun sweeps out equal areas during equal intervals of time." This looks like sweeping a broom over the floor.
In the comic, the janitor Kepler also sweeps the same area, although in this case, "area" is used in the sense of "surface" (of floor) rather than in the purely mathematical sense. It is also very monotonous, like a planet's set orbit, but Kepler doesn't mind this.
The comic could also be seen as a subtle reference to the Kepler space telescope that was searching for exoplanets (planets outside the Solar system) from March 2009 to August 2013, by looking at exactly the same spot in the night sky over and over again. Even though the telescope was not launched until 4 years after this comic was published, the details of Project Kepler had been disclosed by NASA press releases as early as 2001. 
The title text assumes that the reader is scientifically illiterate and won't understand the joke, which is ironic, considering how xkcd came to be known for embracing STEM fields and nerdiness in general.
- [Two Cueball-like guys stand in an aisle in a store.]
- Cueball-like guy: Nice store. How do you keep the floors so clean?
- Cueball the store manager: Oh, we hired this dude named Kepler, he's really good. Hard worker. Doesn't mind the monotony. Sweeps out the same area every night.
- This was the 20th comic originally posted to LiveJournal.
- Original title: "Monday's drawing"
- Original Randall quote: "Another one which, if you don't get, you're probably better off."
- This comic was posted on xkcd when the web site opened on Sunday the 1st of January 2006.
- It was posted along with all 41 comics posted before that on LiveJournal as well as a few others.
- The latter explaining why the numbers of these 41 LiveJournal comics ranges from 1-44.
- This is the same day that NASA announced the delay of Project Kepler due to budget cuts.
- One of the original drawings drawn on checkered paper.
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