If a person is accused of "crying crocodile tears," he or she is thought not to be crying genuine tears, but only faking sadness. The expression has been around for years and comes from observations of crocodile behavior. When eating, crocodiles do often emit tears from tear ducts. Interestingly, scientists recently have shown that crocodiles' tears are caused by a physiological response to the digestion process not by pity, phony or real, for their victims. What is the author's main purpose in the passage?