This joke works because plot means to draw a graph to visualize information, but it also means to scheme, to plan evil, to hatch a sinister plot. The joke works because a math teacher can plot a graph, which could be involved in plotting a crime, but not usually, which makes the speaker seem suspicious, and in a silly way, as if the plan of the graph could be the plan of a crime simply because the word plot is used to say both. It is possible our math teacher is planning a bank heist, with the suspicious graph. The math teacher is certainly plotting something, the graph, and what is normal isn’t suspicious. This shows us the word plot is used by us in two ways, and the difference is fear, suspicion that a plan is more than a plan, it is a plan for evil, and we brace for evil with fear. A plan is someone being calm and resolute in a way, and a sinister plot is a plan, a resolution, that others fear. The turn from a calm, normal, plotted mathematical situation to unreasonable paranoia and aggression is the jerk of the joke. If we look at language use in particular situations from a pragmatic perspective, and keep an eye on the situation of emotions, and how emotions can change, we can understand what jerks us around and makes us laugh at some jokes and not others.
A Zen master posed his monks with a problem: “Two monks went walking in the rain. One didn’t get wet. Why?” The monks suggested that one had an umbrella, that the rain was scattered in places, that one walked under the cover of awnings, but the master said that the students were too focused on the words. When the monks finally gave up, the master told them that both got wet. “Two monks went walking in the rain. One didn’t get wet. Two got wet.”
The joke works just as well in ancient Chinese as it does in modern English because language has grey areas and ambiguities. When the master said, “One didn’t get wet”, he could mean that it is the case that one didn’t get wet, such that one remained dry, or he could mean that it isn’t the case that one got wet, rather two got wet. All of the solutions proposed by the monks assumed that one didn’t get wet, the first case, making them blind to the second. It isn’t that the first case is the literal meaning of the words and the second metaphorical or derivative, but rather that we do not expect to hear about one monk and not the other if both got wet or both stayed dry. The joke would also work if the master said both stayed dry, as one didn’t get wet, and the other didn’t get wet either.