Imagine a 10 gallon tank that is filled by a faucet. We refer to the faucet as the source of water flowing into the tank, S. If S=2.0 gallon per minute (2.0 gal/min) then the tank will become completely filled in 5 minutes. The amount of water in the tank at any given time will be referred to as the tank volume, V.
If we start with an empty tank and turn on the faucet at a constant flow (S in gallons per minute), then the water volume V at any time t is easy to calculate from