In the case of bacteria, exponential growth refers to their unrestrained growth as they proliferate in quantity. Another example is compound interest in finance, which is a fixed rate that grows at a particular percent each year to expand exponentially over time. Exponential decay, on the other hand, can be compared to radioactive decay. This is when materials degrade and waste away in half over time (thus explaining its alias name, half-life material). The second example of exponential decay is a retirement fund that, as it is paid out, usually monthly, begins to diminish in terms of the amount of money left in the fund.