Limits are used instead. With these rules, formulas for a functions can sometimes be more easily obtained from formula for their slopes. In particular, by the use of functions, and anti-differentiation, formulas are obtained in calculus for areas, weights and masses of common shapes and objects. Suppose you are walking or skiing along a smooth path, a path with no vertical steps or drops. The rod should be pivoting at the point or provide a bridge between two points on the hill, on either side of the point where the slope is to be found.
Shorter rods are better for slope approximation than longer ones.
A more precise definition or explanation of how to compute slopes requires the notions of a limit. In flow measurement, there 10 kilograms of matter passing a given point per second. If you are averse to kilograms, replace the 10 kilograms by 21 pounds. Celsius are both temperature increases. The reverse changes would be decreases.