Calculus connects the rate of change of a quantity with the amount of the quantity. Imagine we have a water tank that we can fill or drain. The [i]rate[/i] at which the tank is filled or drained is given in gallons per minute (gpm), and the [i]amount[/i] of water is given in gallons. The fill rate can be changed by sliding any of the red "dots" to create a fill/drain rate "profile" over time.