The Dilemma: You want to assail someone's intelligence, but you don't know quite what word to use, which calls into question your own intellect.
Materials Needed: An IQ test would be helpful, but you can get by on just your wits—provided you have enough of them.
People You Can Impress: Well, idiots and morons both, for starters. But also psychologists. And you really, really need to impress psychologists, because—as you'll see—you really don't want them to think you're an idiot.
Quick Trick: These days, the words are completely synonymous. But back in the dark days of psychology (which is to say until about 30 years ago), there was a difference, and here's the quick trick psychologists used: Ask a question. If your subject answers, they're a moron at worst. If they don't answer, you might have an idiot on your hands.
The Reason: Anyone who says that political correctness never accomplished anything worthwhile should take a long, hard look at the lot of the idiot. In 1911, French psychologists Alfred Binet and Theodore Simon created the first modern intelligence test, which calculated IQ based on whether children could accomplish tasks like pointing to their nose (honestly) and counting pennies. The concept of "IQ" followed soon after, and psychologists fell so deeply in love with the scientific nature of the tests that they created classification systems. Any child with an IQ of above 70 was considered "normal," while kids above 130 were considered "gifted." To deal with kids below 70, psychologists invented a nomenclature of retardation. Those with IQs between 51 and 70 were called morons. Morons had adequate learning skills to complete menial tasks and communicate. Imbeciles, with IQs between 26 and 50, never progressed past a mental age of about six. And the lowest of all were the idiots, with IQs between 0 and 24, who were characterized by poor motor skill, extremely limited communication, and little response to stimulus.
Also Good to Know: The doubly offensive term "Mongolian Idiot," which in the 19th and early 20th centuries was an actual, literal diagnosis, derives from people's belief that individuals with Down Syndrome—with their wide-set eyes and round faces—resembled Mongolians. In fact, before the British physician JHL Down (1829-1896) lent his name to the chromosomal syndrome, Down Syndrome was known merely as "Mongolism."
And there you have it. Now you know whether you're working with idiots or morons. Need more clarification in your life? Check out What's the Difference, a mental_floss book written by John Green, Chris Connolly, Christopher Smith, and (yours truly) Maggie Koerth-Baker.