from a skyscraper onto something soft.

Let's say, a stack of mattresses.

The mouse lands and is stunned for a moment,

before it shakes itself off,

and walks away pretty annoyed,

because that's a very rude thing to do.

The dog breaks all of its bones

and dies in an unspectacular way,

and the elephant explodes into a red puddle of bones and insides

and has no chance to be annoyed.

Why does the mouse survive,

but the elephant and dog don't?

The answer is size.

Size is the most underappreciated regulator of living things.

Size determines everything about our biology,

how we are built, how we experience the world, how we live and die.

It does so because the physical laws are different for different sized animals.

Life spans seven orders of magnitude, from invisible bacteria to mites, ants,

mice, dogs, humans, elephants and, blue whales. Every size lives in its own

unique universe right next to each other, each with its own rules, upsides, and

downsides. We'll explore these different worlds in a series of videos. Let's get

back to the initial question: Why did our mouse survive the fall? Because of how

scaling size changes everything; a principle that we'll meet over and

over again. Very small things, for example, are practically immune to falling from

great heights because the smaller you are the less you care about the effect

of gravity. Imagine a theoretical spherical animal

the size of a marble. It has three features: its length, its surface area,

(which is covered in skin) and its volume, or all the stuff inside it like organs,

muscles, hopes and dreams. If we make it ten times longer, say the size of a

basketball, the rest of its features don't just grow ten times. Its skin will

grow 100 times and it's inside (so it's volume) grows by 1000 times. The volume

determines the weight, or more accurately, mass of the animal. The more mass you

have, the higher your kinetic energy before you hit the ground and the

stronger the impact shock. The more surface area in relation to your volume

or mass you have, the more the impact gets distributed and softened, and also

the more air resistance will slow you down. An elephant is so big that it has

extremely little surface area in ratio to its volume. So a lot of kinetic energy

gets distributed over a small space and the air doesn't slow it down much at all.

That's why it's completely destroyed in an impressive explosion of goo when it

hits the ground. The other extreme, insects, have a huge surface area in

relation to their tiny mass so you can literally throw an ant from an airplane

and it will not be seriously harmed. But while falling is irrelevant in the small

world there are other forces for the harmless for us but extremely dangerous

for small beings. Like surface tension which turns water into a potentially

deadly substance for insects. How does it work? Water has the tendency to stick to

itself; its molecules are attracted to each other through a force called

cohesion which creates a tension on its surface that you can imagine as a sort

of invisible skin. For us this skin is so weak that we don't even notice it

normally. If you get wet about 800 grams of water or about one percent of your

body weight sticks to you. A wet mouse has about 3 grams of water sticking to

it, which is more than 10% of its body weight. Imagine having eight full water

bottle sticking to you when you leave the shower. But for an insect the force

of water surface tension is so strong that getting wet is a question of life

and death. If we were to shrink you to the size of

an ant and you touch water it would be like you were reaching into glue. It

would quickly engulf you, its surface tension too hard for you to break and

you'd drown. So insects evolved to be water repellent. For one their exoskeleton is

covered with a thin layer of wax just like a car. This makes their surface at

least partly water repellent because it can't stick to it very well. Many insects

are also covered with tiny hairs that serve as a barrier. They vastly increase

their surface area and prevent the droplets from touching their exoskeleton

and make it easier to get rid of droplets. To make use of surface tension

evolution cracked nanotechnology billions of years before us. Some insects

have evolved a surface covered by a short and extremely dense coat of water

repelling hair. Some have more than a million hairs per square millimeter when

the insect dives under water air stays inside their fur and forms a coat of air.

Water can't enter it because their hairs are too tiny to break its surface tension.

But it gets even better, as the oxygen of the air bubble runs out, new oxygen

diffuses into the bubble from the water around, it while the carbon dioxide

diffuses outwards into the water. And so the insect carries its own outside lung

around and can basically breathe underwater thanks to surface tension.

This is the same principle that enables pond skaters to walk on water by the way,

tiny anti-water hairs. The smaller you get the weirder the environment becomes. At

some point even air becomes more and more solid. Let's now zoom down to the

smallest insects known, about half the size of a grain of salt,

only 0.15 millimeters long: the Fairy Fly. They live in a world even weirder than

another insects. For them air itself is like thin jello, a syrup-like mass

surrounding them at all times. Movement through it is not easy. Flying

on this level is not like elegant gliding; they have to kind of grab and

hold onto air. So their wings look like big hairy arms rather than proper insect

wings. They literally swim through the air, like a tiny gross alien through

syrup. Things only become stranger from here on

as we explore more universes of different sizes. The physical rules are

so different for each size that evolution had to engineer around them

over and over as life grew in size in the last billion years. So why are there

no ants the size of horses? Why are no elephants the size of amoeba? Why?

We'll discuss this in the next part.

We have a monthly newsletter now, sign up if

you don't want to miss new videos and for bonus videos.