Because this is our video, and we want to see what happens.

First, our now tiny elephant stumbles around and then drops dead.

Tiny elephant buddy is very cold, frozen to death in minutes.

Our giant mouse looks very uncomfortable for a moment, and then it explodes, leaving hot mouse insides everywhere.

Why?

Because of size.

We are optimized to function precisely for the size we are, and would die horribly in any other environment.

But, why exactly?

Why does our mouse explode, and can we do this to our elephant too, if we try hard?

Life on this planet is based on cells.

Cells do vary in size, but they're pretty similar in their dimensions across all species.

A blue whale doesn't have bigger cells than a hummingbird, just a lot more of them.

Cells have to do a lot of stuff to stay alive, and they need energy to be able to do so.

To get this energy, animal cells convert food and oxygen into usable chemical energy.

This happens in our mitochondria, the powerhouse of the cell.

They're like little coal engines that spit out tiny ATP batteries, which the cell can use for almost everything it needs to do.

Just like an engine, mitochondria get really hot while working.

In human skin cells, they reach a scorching 50 degrees Celsius.

And some of our cells have up to 2,000 mitochondria, which are radiating their heat into the cell.

So, being alive generates a lot of heat.

The more cells you have, the more heat your body generates in total.

If our bodies didn't find ways of losing this heat, we would be cooked from the inside and die.

But this is a problem for bigger animals, because of the way bodies change as living beings scale up.

Animals have three properties here that are important.

Their length, their outsides or skin. And their insides, like organs, bones, and hopes and dreams.

The thing that's hard to wrap your head around is that when things grow, their insides grow faster than their outsides.

Imagine a fleshy cube.

If you double the length of its sides, its surface and volume do not double.

In fact, the surface is now four times the original size, and the volume of the cube eight times the original size.

Which is called the Square Cube Law, and has been annoying nature for billions of years.

So why is this a problem for big animals?

Because heat can only leave an object via its surface.

So if we make our mouse the size of an elephant, or 60 times longer, it has 3,600 times more surface from which to lose heat.

But it has 216,000 times more volume filled with trillions and trillions of new hot mitochondria that produce more heat.

A lot more insides, not that much more skin.

Our mouse is very dead, very fast.

But big things like elephants exist.

So how do they deal with the heat?

For one, they evolved ways to get rid of energy more easily, like huge flat ears, that have a lot of surface where heat can escape.

But that's not enough.

Nature's solution is actually very elegant.

Elephant cells are much, much slower than mice cells.

The bigger an animal is, the less active its cells are.

If we classify animals by their metabolic rates, and compare that to their overall mass, it's clearly visible.

It's not 100% accurate, but it is a good rule of thumb.

Elephants are huge meat sacks filled with trillions and trillions of little coal ovens.

So, they keep the ovens just active enough to keep them running and never full power.

Their whole metabolism is slow.

Things move at a nice chill pace.

Small animals need to go the exact opposite way.

If you're small, you have a lot of surface area compared to not a lot of volume.

You don't have a lot of cell ovens, and lose the heat they produce very fast.

So very tiny mammals came up with a very extreme solution.

Meet the Etruscan Shrew, the smallest mammal on Earth.

A mole-like thing that's more closely related to hedgehogs than to mice.

With the body length of four centimeters, it only weighs about 1.8 grams, as much as a paperclip.

It's a tiny, ridiculous being.

It would basically cool off immediately, so its cells run on overdrive to stay warm.

Its tiny ovens are filled at maximum capacity.

Its heart beats up to 1,200 times a minute, and it breathes up to 800 times a minute.

This creates an extreme need for energy, so the shrew has to eat constantly.

After only four hours without food, it starves to death.

And while an African elephant consumes around 4% of its body weight in food each day, our shrew needs 200% of its body weight in food a day just to survive.

Imagine having to eat 2,000 Big Macs a day.

More than one a minute.

Fun for a while, but then not so much.

So, a cubic centimetre of shrew needs 40 times more food than a cubic centimetre of elephant.

If an elephant's cells suddenly become as active as the cells of a shrew, a crazy amount of heat would be generated.

All the liquids in the elephant would suddenly start boiling, and then it would explode in an impressive explosion of steaming hot burning elephant parts.

In reality, before an explosion occurred, the proteins making up our cells would probably be denatured, and stop producing heat.

But a meat explosion is much more fun than melting an elephant into a massive hot goo.

Regardless, the scaling of the speed of metabolism happens everywhere.

Even in places we don't expect, like pregnant women.

A baby in the womb of its mother behaves as if it were a part of her.

Its cells have about the same metabolic rate, the same speed of life, as its mother's organs.

It is truly a part of a bigger whole, rather than an individual.

Until it's not anymore.

The very moment a baby is born, a switch is flipped, and all its internal processes speed up rapidly.

Thirty-six hours after birth, the baby's cells have the same activity rate as a mammal its size.

Babies literally transition from being an organ to being an individual, in mere hours.

But there's one thing where big and small things are very similar:

Heartbeats.

Mammals tend to have a similar amount of heartbeats, over their lifetime.

Typically around one billion.

So, while the shrew and elephant are very different, they share a similar number of heartbeats all the course of their lives.

Their speed of life is the opposite and somehow still the same.

And, for a video in which we made elephants explode for no good reason, this is the most romantic ending we could come up with.