Once in a Blue Moon and Other Idioms That Don’t Make Scientific Sense

Concepts from science and nature pervade our language’s common phrases, idioms and colloquialisms. The incredulous expression”Well, I’ll be a monkey’s uncle” stems from sarcastic disbelief over Darwin’s writings on evolution. To be “in the limelight”—at the center of attention—harks back to how theater stages used to be lit by heating lime (calcium oxide) until it glowed a brilliant white, then focusing the light emitted into a spotlight.

Someone as “mad as a hatter” exhibits behavior similar to 18th and 19th century hat makers who stiffened felt cloth with mercury—an ingredient that after continued exposure causes dementia. “Tuning in” to someone’s message has its origins in the slight turns of a dial needed to focus on a radio signal.

These colorful expressions bring spice to our language. Yet certain well-used phrases from science are misrepresentations of what they’re trying to express. Others are just plain wrong!

Some are obvious, yet we use them anyhow. A person who sagaciously shakes her head and says “A watched pot never boils” while you are waiting second after agonizing second for test results to arrive or job offers to come in knows that if she sat down and watched a vessel containing water on a stove over high heat for long enough, the water will eventually boil. Or the person who utters the placating phrase that “the darkest hour is just before dawn,” meant to give hope to people during troubled times, probably knows that well before the Sun rises, the sky gets progressively lighter, just as how well after the Sun sets, light lingers until the Earth rotates beyond the reach of the Sun’s rays. Thus, the darkest hour of the night (in the absence of the Moon) is midway between sunset and sunrise.

A few phrases, however, have less obvious scientific inaccuracies. Here are a few for you to consider:

1. Once in a blue moon: This poetic phrase refers to something extremely rare in occurrence. A blue moon is the term commonly used for a second full moon that occasionally appears in a single month of our solar-based calendars.  The problem with the phrase, however, is that blue moons are not so rare—they happen every few years at least, and can even happen within months of each other when the 29.5-day lunar cycle puts the full moon at the beginning of any month but February.

The usage of “blue moon” as the second full moon in a month dates back to a 1937 Marine Farmer’s Almanac. But prior to that, blue moons meant something slightly different. Typically, 12 full moons occur from winter solstice to the next winter solstice (roughly three per season), but occasionally a fourth full moon in a season could be observed. In such a case, one of the four full moons in that season was labeled “blue.”

Readers may recall that baby Smurfs are delivered to the Smurf village during blue moons. If this were to occur every blue moon, we’d soon be awash in blue creatures three apples high!

2. Where there’s smoke, there’s fire: The phrase means that if something looks wrong, it likely is wrong. But let’s step back—do you always have to have fire if you see smoke?

Answering that first requires defining “fire.” Merriam-Webster’s first definition of fire is “the phenomenon of combustion manifested in light, flame, and heat.” Combustion is the chemical reaction that occurs when fuel is burned in the presence of oxygen. So for a fire to ignite and be sustained, it needs heat, fuel and oxygen—denying a fire any of these three things will extinguish the fire; attempting to start a fire without one of three things will be futile.

In complete combustion—what occurs when you light a gas stove—the fire produces no smoke. However, when most materials are burned, they undergo incomplete combustion, which means that the fire isn’t able to completely burn all of the fuel. Smoke is an airborne collection of little particles of these unburned materials.

The reason why these materials didn’t burn is because of pyrolysis—the breakdown of of organic material at elevated temperatures in the absence, or under a shortage, of oxygen. Think of it this way: a wood fire’s quick consumption of oxygen depletes the gas’s presence around a burning log, and this localized lack of oxygen while the log is at high temperatures causes log to char, breaking the log down into a substance much richer in carbon content. The resulting charcoal, if still under high heat, can then smolder—a flameless form of combustion—until all the fuel is consumed.

Smoke, then, can be considered to be a product of pyrolysis rather than of fire itself. You’re probably thinking—so what? To get the smoke, a fire needed to be present at some point, right?

Not always. Let’s consider pyrolysis to the extreme. For example, tobacco leaves heated to 800 degrees Celsius in a pure nitrogen atmosphere undergo pyrolysis and release smoke without actually being on fire.

Pyrolysis without fire can also occur in more familiar circumstances. Imagine blackening a piece of fish on a pan using an electric range, where electricity heats metal coils on the cooktop until they are incandescent, but not on fire. Leave the fish unattended for too long and it will start to char and smoke. But why bother with putting fish in the pan? Those looking for fireless smoke need to go no further than melting a slab of butter in a sauté pan. All oils and fats used in cooking have smoke points—the temperature at which they start to degrade into a charred goo of glycerol and fatty acids—as seen in this video.

Sure, leaving these smoking substances on the range for too long will cause them to eventually combust (oils and fats, after all, do have flash points), but before that, you have a whole lot of smoke with no fire!

3. The fish rots from the head down: The phrase seems to pop up more frequently when political scandals or accusations of malfeasance make headlines. The origin of the phrase is murky, likely stemming from folk proverbs of Europe and Asia Minor. But the meaning is simple–if a system is corrupt, its leaders instigated the corruption.

The authoritative ring to this phrase belies its accuracy. Fish, in fact, start to rot from the gut. According to David Groman, an expert on fish pathology at the University of Prince Edward Island, the proverb is a “poor metaphor. And, I must say, it’s biologically incorrect,” he told Anna Muoio of the business magazine Fast Company.  “When a fish rots, the organs in the gut go first. If you can’t tell that a fish is rotting by the smell of it, you’ll sure know when you cut it open and everything pours out–when all the internal tissue loses its integrity and turns into liquid.”

The reporter then got hold of Richard Yokoyama, manager of Seattle’s Pike Place Fish Market, who said “Before I buy a fish from one of our dealers, I always look at the belly. On a fish, that’s the first thing to go. That’s where all the action is–in the gut. If the belly is brown and the bones are breaking through the skin, I toss the fish out. It’s rotten.”

Unfortunately for scientific accuracy, saying “The fish rots from the belly outward” lacks gravitas and is unlikely to be picked up by the punditsphere.

4. Hard as nails: The saying is often used to describe a person who is stern, unyeilding, unsympathetic, bordering on ruthless. An early appearance of the phrase can be found in Dickens’ Oliver Twist, when the Artful Dodger and the other street urchins describe their pickpocketing work ethic.

But let’s take a step back–are nails really that hard? The hardness of a material can be estimated relative to other substances according to where it falls on Mohs scale of mineral hardness. This scale, which ranges from one through 10, was developed by the German geologist in 1812 to help him classify the minerals he encountered in his excursions. Talc, a soft mineral easily powdered, is a one on the scale. The malleable element copper sits at a three. Quartz—the clear crystal common in sand or the spiny lining on the inside of a geode—is a seven. Diamond, the hardest natural substance on the planet, is a 10.

Mohs’ scale is an ordinal scale, which means that it doesn’t estimate the degree to which one substance is harder than another. Rather, it is based on the idea that materials that fall at higher values on this scale can scratch anything with lower numbers, and that materials with low hardness numbers cannot scratch anything with a higher hardness value. On this scale, a  steel nail used to fasten wood together would hit at about 5.5. Feldspars, such as the pink minerals of granite, are harder than those nails, as are topaz, quartz, sapphires and of course diamonds. Even unglazed porcelain, which is about a seven on the scale, is harder than an average nail.

But not all nails are created equally. The nails used in wood are are made of  low-carbon or “mild” steel, meaning that the chemical composition of their alloys are only between 0.05 to 0.6 percent carbon. Nails used to fasten concrete together, for example, have higher percentages of carbon–approaching one percent–which can push the hardness up to as high as a nine on Mohs scale.

So the more correct version of this phrase would be, “Hard as high-carbon steel nails,” but somehow that just doesn’t have the same ring, does it?

5. Diamonds are forever: Thanks to the DeBeers slogan, adorning your honey’s neck, wrists and fingers with bits of pressurized carbon has somehow become a metaphor for true and timeless love. Of course, no object that you can hold in your hand can last forever. But diamonds have a special reason for being incapable of eternity–without the extreme pressures of the deep Earth where they formed, a diamond will slowly revert back into graphite–which is why the older a diamond is, the more inclusions it’s likely to have.

Although it usually will take millions of years for the rock on your finger to become ready for use in pencils, some mineral forms of carbon seem to quickly flash between diamond and graphite depending on the pressures that they are exposed to in the lab. For those mutable sometimes-gems, diamonds are in fact transient.

What common phrases push your buttons when viewed under the microscope of science? Or perhaps you have the inside scoop on whether wet hens really get angry? Let us know!

Mohi Kumar | | READ MORE

Mohi Kumar is a science writer, editor and essayist based in Houston, Texas, and also a freelance editor for Smithsonian.com.