The Dilemma: As far as you know, the only difference between the two is that AM has more preachers.

People You Can Impress: Those preachers, for starters

The Quick Trick: Static is key. If there’s static, it’s AM. Of course, that whole thing about the preachers isn’t a bad rule of thumb either.

The Explanation:
FM stands for “frequency modulation” and AM stands for “amplitude modulation,” so it’s not hard to figure out that the distinction is based on the way a radio wave modulates, or fluctuates. FM waves differ from one another based on frequency, or how many times per second the wave’s current changes direction. AM waves, however, fluctuate based on amplitude, which refers to the specific strength of the signal. All radio waves experience changes in amplitude as they travel, but obviously, if the amplitude isn’t strong enough when it reaches a receiver, you’ll hear static. And because AM waves depend on specific amplitudes to get a signal, they’re less reliable. This also makes them less valuable, which is why it’s easier for all those farm news enthusiasts and mariachi bands to get one of their own.

If you’re wondering about their order, there’s no chicken and egg here: The AM definitely came first. Inventor Reginald Fessenden made the first AM radio broadcast in 1906—presaging those radio preachers by making his first broadcast a reading from the Bible and a live performance of himself playing “O Holy Night” on the violin. And while the medium was most popular from 1920 to the advent of FM in the ’50s, AM certainly has its limitations. For one thing, AM travels by sound waves close to the Earth during the day and higher in the sky in the evening, meaning it’s hard to have a large broadcast radius during daylight hours. For another thing, AM signals, unlike FM, are often disrupted by tall buildings—a bigger problem today than in 1920.

As for FM, it was invented in 1933 by Edwin Armstrong—but sadly enough, Armstrong never lived to see his invention succeed. Convinced FM had failed, Armstrong committed suicide by jumping out of his window in 1954. Just a few years later, the superior sound quality and general betterness of FM were recognized by the American public, and FM took off, making Armstrong’s widow rich.

Popularity Contest
FM has supplanted AM as the most popular world-wide format—if you trust the CIA’s fact-gathering abilities. According to the CIA’s World Factbook, there are about 28,693 FM radio stations in the world and only 16,265 AM stations.