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Hey, Vsauce, Michael here, and bananas are fantastic. They're actually one of the most
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radioactive foods we regularly eat. Sometimes they're difficult to peel from the top. One
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of my favorite ways to avoid that is to simply hold the banana and snap it in half. Boom,
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now it peels itself. It's a wonderful little trick, except when you drop it on the ground.
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This is unfortunate, but, if it picked it up quickly, would it be safe enough to eat?
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I mean, how quickly do the bacteria on the floor move onto the food? Some people call
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it the "5-Second Rule." Others, the "10-Second Rule." And still, others, the "2-Second Rule."
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But, is there any truth to it, or is it just something we say to allow us to eat food off
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the floor?
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In 2003, Jillian Clarke famously investigated this question. She found that 50% of men use
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the 5-Second Rule to eat food off the ground. But, 70% of women do. More importantly, she
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found that even brief contact with a contaminated floor will contaminate food, wet or dry.
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The Myth Busters found similar results. And a paper published in the Journal of Applied
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Microbiology got even more technical. The researchers contaminated various floor surfaces
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with salmonella, and they found that 5 seconds is way too long to wait. Bacteria adhere to
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dropped food almost immediately, but time does matter- after 5 seconds, they found that
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the food had acquired anywhere from 150-8000 bacteria. But, if left for a full minute,
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the number they found was 10 times greater.
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Now considering it only takes about 10 bacteria of certain strains of salmonella to infect
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you, you should probably think twice before eating off the ground.
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Oh, and don't complacently think that you're safe because the floor looks clean, or the
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food that fell on it does. Floors make great homes...for bacteria. Also, floors come in
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contact with the bottoms of our shoes, and the University of Arizona has found that 93%
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of our shoes are contaminated with fecal bacteria.
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I've linked all of these studies down in the description. They're great reads- very thorough.
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But, is it true that food dropped on a dirty floor becomes contaminated immediately? I
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mean, sure, practically speaking, that makes a lot of sense. But instantly? Is there a
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limit for how quickly bacteria can move from one object to another? Is there an amount
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of time, short enough, that were I to pick the food up within it, there would be no way
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for what it touched to contaminate it.
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Well, we're going to need to define "touch." I've talked about this before on Vsauce, and
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on Thinker, and MinutePhysics has a great quantum perspective, but when we say "touch,"
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we tend to think of two objects contacting each other with no space in between. Unfortunately,
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that's not really what happens. At a subatomic level, atoms resist smashing together because
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their electrons repel. Electrons can be modeled as waves, waves that overlap and interact,
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but they never touch. There's always space between them. When I touch something, or someone,
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I'm really just feeling their electrons react to mine at a distance. A sub-atomically small
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one, but a real one.
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So, if that's the case, how come glue can stick things together? How come lipstick sticks
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to lips? And how can bacteria on a dirty floor stick to food that's been dropped on it?
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The positive charge from the protons in an atom or molecule isn't always balanced perfectly
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in space by the negative charge from electrons. Sometimes a molecule is asymmetrical and this
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imbalance is permanent, which gives the molecule a constant dipole, so it's like a tiny little
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magnet.
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But even in symmetrical molecules, electrons are mobile. At any one point in time they
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might happen to find themselves more towards one end of the molecule than the other, creating
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rapidly fluctuating dipoles, causing the molecule to act like a magnet.
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If molecules have a lot of energy they will simply whiz by one another, and their dipoles
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and those of their neighbors won't matter much. But, if you cool them down, slow them
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down, the dipoles will have more of an effect, which is why molecules tend to stick together
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as they cool- going from a gas, to a liquid, to a solid.
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That's a lot of information about molecules, but it's incredibly important. It's the reason
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things can get wet. In fact, when you get out of the shower, you, on average, weigh
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about one more pound more than you did before you got wet. It's also the reason Capillary
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Action happens. If you dip a napkin in a glass of water, you can watch water climb up the napkin
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against gravity. That's because the molecules of water have very strong dipoles, and they're
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attracted to each other (cohesion), and they're attracted to molecules of the paper (adhesion),
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more strongly than gravity pulls them down.
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If you want to learn more about intermolecular forces, I've put a bunch of links down in
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the description. But here's my question: how quickly do those forces act? Can two surfaces
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come into contact, briefly enough, that their molecules don't have time to be influenced
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by intermolecular forces.
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Well, for this, we're going to need Molecular Dynamics.
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Molecular Dynamics is the computer simulation of the physical movements of atoms or molecules.
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For instance, this is a simulation of a single Copper atom depositing on a Copper surface.
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Notice that the timer up there is counting off hundredths of a picosecond. These simulations
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need to have a narrow enough time-step to account for the fastest molecular vibrations
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of the material, including everything from wagging, to scissoring.
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Typically, time as brief as a quadrillionth of a second is taken into account. So, the
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5-second Rule may be true...If we rename it the "1 Femtosecond Rule." Spend less time
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on the floor than that, and it's unlikely that room temperature molecules would have
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time to be influenced by intermolecular forces. Except, oops, I think we've been thinking
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too small. Intermolecular forces are fun, but objects can become entangled with each
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other on a macroscopic level.
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Glue, and makeup, and other sticky stuff often take advantage of the tiny imperfections-
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ridges, and nooks and crannies- on an objects surface. Sticky things can seep inside and
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hold on. Even surfaces that seem smooth to us, when you really get down to it, aren't
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smooth at all.
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Two sheets of paper from a phonebook may slide across each other quite easily, but multiply
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that friction by the number of pages in two phonebooks by interweaving the pages, and
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you've got yourself a monster capable of lifting an entire car.
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I was recently lucky enough to see this happen in real life. You can check out my adventure
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over at Fast, Furious, and Funny. It's a new channel, stay tuned because we lifted cars
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with more than just phonebooks...
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The adhesion between two objects caused by the shapes of their surfaces contributes to
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friction and is known as "mechanical adhesion." It plays a pretty big role in getting floor
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germs to sick to dropped food.
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In fact, if two surfaces can mechanically adhere, but aren't close enough to do so,
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we wouldn't say they were touching. And so, if by definition "touching" means that you
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can mechanically adhere to other object's surface, well, then, maybe the 5-Second Rule shouldn't
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be known as the "1 Femtosecond Rule," because if they're touching, it's already too late,
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and, instead, the rule should be known as the "Don't Touch Food That's Fallen on the
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Floor...Rule."
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But come to think of it, maybe we shouldn't be eating food that's touched anything because
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bacteria are everywhere, including on you right now. There are more bacteria on your
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body right now than there are people living in America.
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There are 40 million bacteria in one gram of soil, and 5 Nonmillion bacteria on Earth.
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It's been found that one out of every 10 bank cards, and one out of every 7 bills, has fecal
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bacteria on it.
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Here's something fun: go down to the comment section and type the letter "V." If you used
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a mobile phone to do that, congratulations, you just touched 6,281 bacteria. If you're
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using a desktop keyboard, you're a little safer. You probably only touched about 180.
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Cellphones are actually one of the most bacteria-ridden things we frequently encounter.
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If you want to estimate about how many bacteria are specifically on your phone, based on how
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clean you are, The Oatmeal has a really great quiz- you can find that in the description.
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But honestly, what amazes me the most isn't how dirty our world is, or how much bacteria
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there is; instead, it's the fact that despite those numbers, we don't get sick more often.
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Our immune system is amazing, but it relies on the same principles of adhesion that bacteria
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do.
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The forces that cause food dropped on the floor to pick up germs are the same forces
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that we need to fight them. They're same forces that bring us closer together, whether we
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like it or not.
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So, keep adhering, and, as always, thanks for watching.