The amount of water in the tank is strictly confined to a constant rate of flow. No matter how much is in the tank, the next minute will bring another 10 gallons. Making a graph of the volume over time a straight line. Making the function linear.
An exponential function, like the one governing the growth of a colony of bacteria, would grow at an ever increasing rate. If it doubled every 24 hours, and you started the day with 1,000 bacteria, you would end the day with 2000, for an increase of 1,000. The next day the colony would grow from 2,000 to 4,000 for an increase of 2,000. The rate of growth is a fucnction of the population and time, not time alone, hence exponential.
