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In 1958, an MIT student named Oliver Smoot measured a bridge as part of a fraternity pledge. But he wasn’t allowed to use a ruler or a tape measure.

He had to use himself. He laid down at the start of the bridge, a couple of students marked where his head and his feet were, and then he moved over and laid down again. After an hour and a half of this, the results were in: The bridge was 364.4 smoots long -- plus or minus an ear, because measurements are meaningless without error bars.

That’s about 620 meters, by the way, for those of you in backwards countries that haven’t switched to smoots yet. … Of course, smoots aren’t an official unit, even though Smoot later became chairman of the American National Standards Institute, where part of his job was to define and standardize the units Americans use. Yet, there are plenty of units used every day that can seem just as weird. And even if you’ve never heard of them, you’re benefiting from obscure units every time you put on your shoes or read about dark matter.

People were measuring things long before the French Revolution brought the metric system thundering across Europe. It was easiest to use something common and familiar, so unsurprisingly, a lot of older units came from farming. Like the furlong.

It started as the length of land that an ox could plough without resting, which apparently tended to be about 200 meters. But there were no universal standards — I mean, for one thing, every ox is different — so different places ended up with different-length furlongs. Today, you don’t really hear much about furlongs.

That is, unless you’re into horse racing, where the track stretches for one furlong. Horse heights are also still measured in units of hands instead of centimeters or inches. A hand is exactly a third of a foot, in case you were wondering, or about 10 centimeters.

You’d think measuring in feet would work just as well, at least in the US, but I guess equestrians find it handy. If you look closely, though, you’ll find furlongs hidden all over the place. City blocks in Salt Lake City, Utah, stretch one furlong.

And in the United States, it’s pretty common to hear about land measured in acres, which are just furlongs in disguise. An acre was once the land area that a team of oxen could plough in a day, so acres were originally thin rectangles a furlong long and about a tenth of a furlong wide — although the different furlongs in different places lead to very slightly different acres on either side of the Atlantic. Both are a little more than 4000 square meters.

Even though acres started as thin rectangles, today you’ll hear something described in acres no matter what shape it is. And whenever you do, you’re hearing one of the last hurrahs of the ancient furlong. Have you ever wondered about the logic behind shoe sizes?

I mean, a size 9 isn’t nine centimeters or nine inches or nine meters -- and it’s not like a size 9 is a inch or an centimeter longer than a size 8. Well, if you’re in an English-speaking country like the US or Australia, the barleycorn is your answer. When animal-based measurements were a little too big, people looked for smaller things to measure with.

And in England, they settled on the barleycorn: a single grain of barley. A barleycorn is about eight and a half millimeters long—roughly a third of an inch. But originally, it was exactly a third of an inch -- because an inch, by definition, was three barleycorns long.

Today’s inches, though, are ultimately defined through the speed of light, and measuring with barleycorns is mostly a thing of the past. Except that shoemakers didn’t get the message. Because even if they don’t use actual barleycorns any more, shoe sizes are still based on that ancient unit.

There’s a lot of variation between countries when it comes to details like how much wiggle room your feet get and where size numbers start, but anyone with the English system uses the barleycorn. Kids’ sizes generally start at size 0, which fit adorable tiny feet that are eleven barleycorns long. Then, each size is one barleycorn larger, up to 24 barleycorns in length, or size 13.

Somewhere around there, the adult sizes start, and again each one is one barleycorn bigger than the last. There are other systems in other parts of the world; European shoe sizes, for instance, get about two-thirds of a centimeter longer with each size. But if you’re using the English system, you have the barleycorn to thank if your shoes fit -- or to blame, I guess, if they don’t.

The micromort was first detailed in a 1989 paper by Ron Howard. No, not that Ron Howard. A Stanford professor named Ronald A.

Howard was looking for a concrete way of discussing the risks of certain actions. Howard proposed that if something has a one-in-a-million chance of killing you, it carries one micromort of danger. Which means that instead of being defined by convention or universal constants like the rest of this list, the micromort is defined by statistics.

And that means that something’s micromort count will change over time. Back in 2000, going skydiving in the US would add about 11.9 micromorts to your day -- because there were 32 skydiving fatalities that year, out of 2.7 million jumps. But by 2016, that number dropped to about 6.5 micromorts per jump -- because there were more jumps and fewer deaths.

The count also changes based on where you are. Generally, running a marathon exposes you to about 7 micromorts. But if you run in a smoggy city where it’s hard to breathe, that number can go up.

Which, yes, makes running a marathon more dangerous than skydiving these days. If you’re micro-morbidly curious, you can find all sorts of lists and tables online with micromort counts of different activities. And even though micromorts only measure the risk of death instead of the risks of any sort of injury, they can still help inform your decisions.

Lots of people won’t swim in the ocean because they’re afraid of sharks, for example. And in Australia, one of the countries with the most recorded shark attacks, swimming in the ocean has about 12.125 micromorts of risk. But 12 of those 12.125 micromorts have nothing at all to do with sharks; they’re from the risk of drowning -- which is something people tend not to worry about even though its risk is almost a hundred times higher.

And the 0.125 that is from sharks is about the same as what you get from kangaroos. And both of them combined are less risky than sitting on a chair in Australia, due to the likelihood of dying when you fall off of it—which kind of puts things in perspective. Maybe you’ve heard that the jiffy started as a real unit of time, just like the second.

But what really happened is actually more complicated than that. “Jiffy” used to just mean any short amount of time. Like, “be back in a jiffy!” But since science and engineering have lots of fast things, people in different fields all reached for this fun-sounding word to describe something fast. Physicists love the speed of light, so one common definition you’ll hear is that a jiffy is the time it takes light to go a centimeter in a vacuum.

That’s about 33 picoseconds, or 33 trillionths of a second. Which definitely qualifies as “fast.” But 33 picoseconds isn’t the only jiffy in physics. Other physicists might say a jiffy is a hundredth of a second, since that’s a convenient amount of time for measurements.

And others would say it’s closer to three trillionths of a trillionth of a second -- roughly the time it takes light to go the length of an atomic nucleus. It all depends on what they’re talking about, since no one ever bothered to standardize things. And the trouble doesn’t stop with physicists.

Electrical engineers tend to care way more about cycles of the power from an outlet than they do about the speed of light, so they started saying a jiffy is one full cycle of that power. Different countries have different cycles, making an electrical jiffy 1/60 of a second in the US and 1/50 of a second in Europe. And in computer science, processor cycles are important -- so a jiffy can be the time it takes a computer to complete one computation.

Jiffies aren’t really used for anything official, so the confusion doesn’t cause too much trouble. But the next time someone tells you they’ll be back in a jiff, you might want to ask if they mean a physics, engineering, or computer science jiffy. Just to be safe.

In pictures, it’s good to include a banana for scale. But bananas aren’t just good for measuring size. They’re also pretty good for comparing exposures to radiation.

The banana equivalent dose, or BED, is how much radiation you’ll get from radioactive atoms in your average banana. Yup. That radiation happens because a small fraction of Earth’s potassium is an isotope called potassium-40.

Potassium-40 radioactively decays, so eating a banana means exposing yourself to a tiny bit more ionizing radiation than not eating one. Roughly 1 BED of ionizing radiation comes to about a ten-millionth of a sievert -- the usual unit for radiation. One BED is a really tiny amount.

Each day, you’re naturally exposed to about a hundred BEDs just from rocks and bricks and being out in the world. And a hundred BEDs a day still isn’t worth worrying about -- so there’s absolutely no reason to stop eating bananas just to avoid radiation. They’re delicious.

Even though one BED is so little radiation that it basically doesn’t matter, it can still help us understand the risks of different behaviors. An international flight, for example, exposes you to about four hundred BEDs. And medical tests like X-rays and mammograms can range anywhere from hundreds to tens of thousands of BEDs, depending on the test.

That’s relatively safe as long as you aren’t having them too often, but the medical professionals in the room do sometimes use protective lead walls, since they’re conducting these tests all the time. Radiation therapy, on the other hand, is around twenty million BEDs. Now that’s a lot -- but then again, the whole point of radiation therapy is to kill cells.

That’s why most types of radiation treatment involve keeping the radiation to as small a part of the body as possible. A fatal dose of radiation, meanwhile, is up around a hundred million BEDs. Now, even though they’re convenient — and kind of hilarious — BEDs aren’t a perfect unit.

Potassium-40 doesn’t give off all types of ionizing radiation, and different types can have different effects on your body. Plus, radiation has different effects inside and outside the body. But next time someone talks about radiation, converting into bananas can help you know what they’re talking about.

Atomic bombs and nuclear power both rely on chain reactions of neutrons running into atoms like uranium, which lets off more neutrons. So as you might imagine, the scientists working on the first atomic bomb were pretty interested in just how often neutrons would run into atoms. And since they were always talking about how big uranium atoms were and whether uranium was easy to hit or not, they invented a new unit that was about the size of a uranium nucleus: The barn.

The name came from the fact that trying to hit a big, bulky uranium atom was a bit like trying to hit the broadside of a barn … or something like that. But weird name or not, it stuck as a unit of cross-sectional area. One barn is equal to about a tenth of a trillionth of a trillionth of the size of the “Play” button on this video.

Assuming you are watching on the computer. Nuclear and atomic physicists are always talking about particles hitting or missing or interacting with atoms, so barns still naturally come up all the time. And barns are even out at the frontiers of knowledge.

We still don’t know what dark matter is, but a lot of people think it might occasionally ram into atoms of regular matter -- the stuff you and I are made of. And when they’re talking about the chance of an atom getting hit by dark matter, real physicists write real papers with units like “millibarns” in them. We live in a very strange world.

But as strange as these units are, they do make it easier to talk about things. That’s why units exist. And while here on SciShow, we almost always use metric units plus common oddballs like light-years because they’re what most people worldwide understand, you never know when you’ll need to measure something in furlongs, or in barleycorns.

Or, even, in smoots. Thanks for watching this episode of SciShow, brought to you by our patrons on Patreon. If you want to help support us while getting awesome perks, you can head on over to Patreon.com/scishow. [♩ OUTRO ].