Often in physics my textbook refers to a "surface of constant X" where $X$ is some quantity like temperature, pressure etc.
However, it goes on to refer to the gradient quantity $\partial_i X$, where $\partial_i$ denotes the derivative with respect to some spatial coordinate. I don't understand why this derivative is not just $=0$?
I think my confusion is related to the definition of a level surface e.g. this question, but having scoured the available answers I still do not quite understand.