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In 1977, the physicist Edward Purcell

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calculated that if you push[br]a bacteria and then let go,

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it will stop in about[br]a millionth of a second.

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In that time, it will have traveled less[br]than the width of a single atom.

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The same holds true for a sperm[br]and many other microbes.

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It all has to do with being really small.

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Microscopic creatures inhabit[br]a world alien to us,

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where making it through an inch of water[br]is an incredible endeavor.

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But why does size matter[br]so much for a swimmer?

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What makes the world of a sperm[br]so fundamentally different

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from that of a sperm whale?

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To find out, we need to dive[br]into the physics of fluids.

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Here's a way to think about it.

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Imagine you are swimming in a pool.

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It's you and a whole bunch[br]of water molecules.

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Water molecules outnumber you[br]a thousand trillion trillion to one.

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So, pushing past them[br]with your gigantic body is easy,

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but if you were really small,

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say you were about the size[br]of a water molecule,

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all of a sudden, it's like you're swimming

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in a pool of people.

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Rather than simply swishing by[br]all the teeny, tiny molecules,

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now every single water molecule

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is like another person[br]you have to push past

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to get anywhere.

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In 1883, the physicist Osborne Reynolds

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figured out that there is[br]one simple number

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that can predict how a fluid will behave.

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It's called the Reynolds number,

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and it depends on simple properties[br]like the size of the swimmer,

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its speed, the density of the fluid,

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and the stickiness,[br]or the viscosity, of the fluid.

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What this means is that creatures[br]of very different sizes

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inhabit vastly different worlds.

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For example, because of its huge size,

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a sperm whale inhabits[br]the large Reynolds number world.

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If it flaps its tail once,

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it can coast ahead[br]for an incredible distance.

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Meanwhile, sperm live[br]in a low Reynolds number world.

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If a sperm were to stop flapping its tail,

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it wouldn't even coast past a single atom.

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To imagine what it would[br]feel like to be a sperm,

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you need to bring yourself down[br]to its Reynolds number.

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Picture yourself in a tub of molasses[br]with your arms moving

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about as slow as the minute[br]hand of a clock,

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and you'd have a pretty good idea[br]of what a sperm is up against.

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So, how do microbes[br]manage to get anywhere?

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Well, many don't bother swimming at all.

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They just let the food drift to them.

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This is somewhat like a lazy cow

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that waits for the grass[br]under its mouth to grow back.

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But many microbes do swim,

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and this is where those[br]incredible adaptations come in.

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One trick they can use[br]is to deform the shape of their paddle.

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By cleverly flexing their paddle

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to create more drag on the power stroke[br]than on the recovery stroke,

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single-celled organisms like paramecia

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manage to inch their way[br]through the crowd of water molecules.

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But there's an even more[br]ingenious solution

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arrived at by bacteria and sperm.

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Instead of wagging[br]their paddles back and forth,

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they wind them like a cork screw.

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Just as a cork screw on a wine bottle

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converts winding motion[br]into forward motion,

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these tiny creatures[br]spin their helical tails

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to push themselves forward

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in a world where water[br]feels as thick as cork.

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Other strategies are even stranger.

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Some bacteria take Batman's approach.

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They use grappling hooks[br]to pull themselves along.

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They can even use this grappling hook

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like a sling shot[br]and fling themselves forward.

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Others use chemical engineering.

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H. pylori lives only[br]in the slimy, acidic mucus

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inside our stomachs.

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It releases a chemical[br]that thins out the surrounding mucus,

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allowing it to glide through slime.

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Maybe it's no surprise

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that these guys are also responsible[br]for stomach ulcers.

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So, when you look really closely[br]at our bodies and the world around us,

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you can see all sorts of tiny creatures

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finding clever ways to get around[br]in a sticky situation.

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Without these adaptations,[br]bacteria would never find their hosts,

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and sperms would never[br]make it to their eggs,

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which means you would never[br]get stomach ulcers,

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but you would also never be born[br]in the first place.

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(Pop)