Previous: The Secret Behind Bioluminescent Bays
Next: The Science of Flint's Water Crisis



View count:259,649
Last sync:2022-11-10 04:00
Go to to get $5 off your first purchase!

This week in SciShow News, there's a new kilogram in town, and we might be closer to understanding why people love coffee so much!

Hosted by: Hank Green

SciShow has a spinoff podcast! It's called SciShow Tangents. Check it out at
Support SciShow by becoming a patron on Patreon:
Dooblydoo thanks go to the following Patreon supporters: rokoko, Alex Hackman, Andrew Finley Brenan, Lazarus G, Sam Lutfi, D.A. Noe, الخليفي سلطان, Piya Shedden, KatieMarie Magnone, Scott Satovsky Jr, Charles Southerland, Patrick D. Ashmore, charles george, Kevin Bealer, Chris Peters
Looking for SciShow elsewhere on the internet?
Thanks to for sponsoring this episode.

Right now, is offering SciShow viewers 5 bucks to spend on your first purchase with Stick around to see how you can get 5-bucks for free.

Of all the things that happened last week, one event was particularly, uh, /weighty/. On November 16th, hundreds of scientists from around the world met in Versailles,. France at the 26th meeting of the General Conference on Weights and Measures — because yes, we have needed that many of them.

And they unanimously voted to redefine the kilogram mathematically, tying it to the immutable laws of the universe and democratizing its mass in one fell swoop. The kilogram has been the unit of mass used by the international system of units, aka the metric system, for 129 years. In 1889, scientists decided that a kilogram was the weight of a specific, ping-pong ball-sized metal cylinder they called the international prototype kilogram, though it became known as Le Grand K.

This standardized mass measurements around the globe, but it came with a risk: it was really hard to make sure the cylinder didn’t gain or lose any mass at all. And anything that might change its mass even the slightest bit would literally change the definition of a kilogram, throwing off physics formulas and the calibrations of scales worldwide. Even under lock and key,.

Le Grande K has lost an estimated 50 micrograms, or millionths of a gram, in the last 129 years. While that’s only about the mass of an eyelash, it’s a lot in today’s nanoscale world, where everything from effective drug doses to computer parts can weigh much less than that. So the scientists in charge of maintaining the metric system wanted to find a less variable definition for the kilogram.

And that brought them to Planck’s constant, a tiny number that describes how the energy of a photon relates to its frequency. You can mathematically connect mass to Planck’s constant, which means you can derive what a kilogram is in terms of it— which is how the kilogram will be defined from May 20, 2019 onwards. Scientists wanted to make this change for awhile, but hadn’t figured out the value of Planck’s constant precisely enough.

That all changed last year, thanks to a machine called a Kibble balance. No relation to the type of dog food. A Kibble balance is basically a scale, but it uses an electromagnet to counterbalance whatever you’re weighing, so it essentially weighs things in terms of energy— and therefore Planck’s constant.

And after next year, all of the base standard measurement units will be based on fundamental properties of the universe instead of physical objects, so they won’t ever change again— unless the universe does, I guess. Hopefully, Le Grand K isn’t too bitter about its forced retirement. And speaking of bitterness, in other news last week, scientists have finally figured out why so many people like that bitter brown liquid fuel we call “coffee”.

It’s not that they don’t taste its bitterness. Quite the opposite actually, according to a paper published last week in the journal. Scientific Reports.

Being more sensitive to the bitter flavor of caffeine is actually correlated with increased coffee consumption— probably because they learn to associate the taste of the beverage with caffeine’s stimulant effects. It’s long been thought that people’s preferences for bitter foods or drinks were unconsciously steered by their DNA through variations in taste receptors, the proteins that sense different bitter and sweet compounds. So, for example, people who taste bitterness more strongly might avoid bitter things like brussel sprouts or coffee.

But when studies have tried to connect taste sensitivities to what people eat and drink, they’ve gotten mixed results. That might be because directly measuring variations in taste is a lot of work, so the studies haven’t had enough participants to separate innate effects from learned behaviors. For this latest paper, an international research team decided to use a massive dataset of over 438,000 people from the United Kingdom— a couple orders of magnitude bigger than other studies to date.

They wanted to compare genetic variants associated with the perception of bitter tastes to people’s consumption of bitter drinks, like coffee and tea. The researchers figured people whose genetics suggested they would taste bitter substances more strongly wouldn’t like the taste of bitter drinks in general, so they’d drink less of them. And if they did drink them, they’d pick tea over coffee, since it’s the less bitter of the two.

That held true for predicted sensitivities to the bitter compounds quinine, which is found in coffee in small amounts, and propylthiouracil, an artificial bitter compound. People whose genes suggested sensitivity to them /were/ less likely to drink coffee and more likely to drink tea. But weirdly enough, being able to taste /caffeine’s/ bitterness, specifically, had the opposite effect.

The most sensitive genetic variants were associated with about a 20% increase in the likelihood of downing 4 or more cups of coffee a day. Which seems weird, until you consider how much of what we do is motivated by reward. The authors suggested that the buzz you get after drinking caffeine can act as a positive reward.

And the stuff isn’t as bitter as things like quinine, so it doesn’t make the drink completely unpalatable. That allows people who like caffeine’s stimulant effects learn to like the taste that goes with it. As for why the strongest caffeine tasters don’t drink more tea, the researchers think that might be because coffee is a more efficient way to get caffeine.

The results suggest that the interplay between genes, taste, and behavior is a lot more complex than we first thought. So determining how DNA affects diet and food preferences is going to take a lot more research. But anecdotal evidence-wise, this is all perfectly consistent my aversion to coffee.

Thanks to a very memorable test for an episode we did a few years ago,. I know I’m a supertaster, which is linked to being able to taste bitterness more strongly, including from caffeine. But I don’t like how caffeine makes me feel, so to me, coffee is just a bitter drink with no benefits.

Something else that leaves a bitter taste in my mouth: hearing about yet another hack that means my banking information might have been stolen. And that's where can be a huge help. really is all about keeping your financial information private/ — and it’s free!

The service lets you generate a brand new virtual card number for every purchase you make online, which protects your actual card from hackers and identity thieves. You can make as many cards as you want, and you have the option to set a maximum spending limit for each of them. And it’s super easy!

All it takes is a few clicks when you go to make your online purchase. You can even install a browser extension that makes the process even smoother! Literally, with the extension all you have to do is click this button and it will generate a whole new card for you.

There. Done. New card made.

Right now, SciShow viewers can go to for something extra: $5 to spend on your first purchase. If you like FREE MONEY go to or click on the link in the description to sign up!