Laws
Explanation
The Joke
The comic presents three panels, each depicting a different scientific discipline's relationship with "laws." In the first panel, labeled "Physics," a distinguished-looking older scientist confidently declares: "There are 3 laws and then you can derive everything else." In the second panel, labeled "Biology," a biologist stares in bewilderment and asks: "Laws? For SCIENCE?" In the third panel, labeled "Chemistry," a chemist explains: "There are 4000 laws which always work as long as you're not in one of the 29,000 situations where they don't."
The comic humorously contrasts how different sciences relate to the concept of universal laws. Physics has a small number of elegant, fundamental laws (like Newton's three laws of motion) from which everything else can be derived. Biology is so messy and full of exceptions that the very concept of universal "laws" seems foreign. Chemistry falls somewhere in between -- it has plenty of rules, but they are riddled with so many exceptions that the exceptions vastly outnumber the rules themselves.
The Humor
The comedy comes from the escalating absurdity across the three panels and the accurate characterization of each discipline's culture. Physics' smug confidence in its elegant simplicity is a well-known stereotype -- physicists often pride themselves on deriving complex phenomena from a handful of fundamental principles. Biology's bewildered reaction captures the reality that biological systems are so complex, variable, and contingent on evolutionary history that neat universal laws are nearly impossible to formulate. Chemistry's response is the funniest because it captures the frustration of a discipline that desperately wants to have laws like physics but keeps running into exceptions -- the ratio of 4,000 laws to 29,000 exceptions is a perfect encapsulation of this predicament.
References
The "3 laws" in the physics panel most likely refer to Newton's three laws of motion, which are foundational to classical mechanics. The comic also implicitly references the famous "physics envy" phenomenon -- the tendency for other sciences to wish they had the clean, mathematical elegance of physics. The xkcd comic "Purity" (which ranks fields from sociology to physics by "purity") explores a similar theme. In biology, there are very few universally accepted "laws" -- even apparent rules like "all life uses DNA" have exceptions (RNA viruses). Chemistry's characterization reflects the reality of rules like solubility rules, periodic trends, and reaction predictions, all of which come with extensive lists of exceptions.