Mathematical terms can be useful to describe
interpersonal situations as well.
When you're trying to get to know someone of the opposite sex, it can extend through a series of conversations where you just talk about totally random stuff. During this sequence, each person involved will generally give some implied signs of interest in what the other is talking about.
If the friendship is to go anywhere into the romantic territory, these signs generally don't stop for a while. If it's not going anywhere, though, and one side decides it's time to subtly put an end to things, the last such conversation will have a point where it is suddenly clear that the other person doesn't give a rat's ass what you're talking about, and is only perpetuating the conversation for the sake of politeness.
Maybe the shape of the conversation doesn't immediately change that much, just like the slope of the graphed equation doesn't change much when the point of inflection is passed. But things are about to take a turn for the worse, much like that graph turning over.