Part of the precipitate joke
A chemistry professor couldn't resist interjecting a little philosophy into a class lecture. He interrupted his discussion on balancing chemical equations, saying, "Remember, if you're not part of the solution, you're part of the precipitate!"
(A solution in chemistry is a liquid containing dissolved substances. Precipitates are solid particles in a solution that arise as a result of a chemical reaction. The sentence makes perfect sense because precipitates in a liquid are not considered part of a solution because they are not dissolved.)