“If you’re not part of the solution, you’re part of the precipitate.”
When liquid chemicals react to form a solid product, chemists call this solid product a precipitate; in a precipitation reaction, a precipitate will appear within a liquid chemical solution. Thus, in carrying out such a reaction, we begin with liquid reactants, but in the end, we have both liquid and solid products: the solution and the precipitate.
My informant learned this “nerd joke” from interacting with other teenagers inclined toward science, mathematics, and engineering. He reports that he knows science jokes from science clubs, classes, and competitions; also, his ninth grade chemistry teacher, Mr. Bausback, would often tell science jokes. Thus, he does not remember exactly where he learned this particular joke.
My informant appreciates this joke because of its “nerdiness,” but also says that he finds it funny partially because he knows that most people would not understand or appreciate it—in high school, being a nerd almost means being in a special club. Thus, jokes like this may well reflect the social pressures that high school students face; less academically-inclined students tend to be more “popular” or well-liked than the “nerds,” causing the “nerds” to band together and experience an increased sense of kinship with one another. Since this joke excludes people who do not know much about chemistry, understanding this joke—and other jokes such as this one—is part of belonging to the social circle of nerds.