<p>Total entropy of a thermodynamic system consists of two components: thermal entropy due to energy, and residual entropy due to molecular orientation. In this article, a three-step method for explaining entropy is suggested. Step one is to use a classical method to introduce thermal entropy <em>S<sub>TM</sub></em> as a function of temperature <em>T</em> and heat capacity at constant pressure <em>C<sub>p</sub></em>: <em>S<sub>TM</sub></em> = <em>∫(C<sub>p</sub>/T) dT</em>. Thermal entropy is the entropy due to uncertainty in motion of molecules and vanishes at absolute zero (zero-point energy state). It is also the measure of useless thermal energy that cannot be converted into useful work. The next step is to introduce residual entropy <e...