and they bump into each other.

並且相撞

They'll just shake it off and walk on.

他們會甩開對方，然後繼續行走

Sometimes that happens with molecules too.

這情況有時也會發生在分子身上

They just bounce off each other, and that's that.

他們就只是這樣互相反彈

But what if two people were to bump into each other,

但是如果兩個人撞到對方後

and during that collision,

其中一個人的手

one person's arm got severed

黏到另一人面上

and reattached to the other person's face?

情況又如何呢？

Now that sounds really weird,

聽起來好像很荒謬

but it's similar to one of the many ways

但這與其中一種

that molecules can react with each other.

分子反應相似

Two molecules can join and become one.

兩粒分子結合成一粒

One can split apart and become two.

一粒分裂為兩粒

Molecules can switch parts.

某些部份互相對調

All these changes are chemical reactions,

以上全部都是化學反應

and we can see them happening around us.

它們其實就發生在我們身邊

For example, when fireworks explode,

例如燃燒煙花、

or iron rusts,

鐵生鏽、

or milk goes bad,

牛奶變壞、

or people are born,

人的出生、

grow old,

變老、

die,

死亡

and then decompose.

最後分解

But chemical reactions don't just happen willy nilly!

但是化學反應並不是平白無故地發生

Everything has to be right.

要多方面配合

First, the molecules have to hit each other

首先，分子要以適合的角度

in the right orientation.

互相撞擊

And second, they have to hit each other hard enough,

另外，撞擊必需夠強

in other words, with enough energy.

換言之，需要足夠的能量

Now you're probably thinking

現在你可能會想

that a reaction just happens in one direction and that's it.

化學反應就只會沿一個方向發生

Sometimes that's true.

有時這是正確的

For example, things can't unburn

例如東西不能夠逆燃燒

or unexplode.

或是逆爆炸

But most reactions can happen in both directions,

但大部份的化學反應可以沿兩個方向發生

forward and reverse.

向前和向後

There's no reason that our face-arm guy

正如那個臉上有手臂的男孩

can't bump into armless girl,

可以透過與斷臂女孩的相撞

reattaching that arm back to its original socket.

而將那隻手臂接回原來的臼上般合理

Now let's zoom out a bit.

現在我們將鏡頭拉遠

Now let's say that you've got

假設現在有

a thousand people on the street,

一千人走在街上

and all of them start with their limbs

他們的四肢

normally attached.

都在正常的位置上

At the beginning, every collision is a chance

他們每一次相撞都會

for Person A to transfer an arm to Person B's face.

使路人甲的手臂黏到路人乙的臉上

And so at the beginning,

所以一開始

more and more people end up

會有愈來愈多人的臉

with arms attached to their faces or arms missing.

黏到別人的手又或者失去了自己的手

But as the number of people with arm-faces

因為這些人的數目增加

and missing arms grows,

他們相撞的可能性

collisions between those people become more likely.

亦大大增加

And when they bump into each other,

那麼當他們相撞

guess what?

猜猜發生什麼事？

Normal-appendage people are reproduced.

正常的人會再次產生

Now the number of limb transfers per second forward

這樣說每秒鐘肢體脫離的數目（前行）

will start high and then fall,

一開始是多然後漸少

and the number of limb transfers per second backward

同時每秒鐘肢體回到原位的數目（後行）

will start at zero and then rise.

會由零逐漸上升

Eventually they'll meet,

最後它們會相遇

they'll be the same.

而且會是相同

And when that happens,

當這發生

the number of people in each state stops changing,

每種形態的人的數目將會停止改變

even though people are still bumping into each other

雖然他們仍然會不斷相撞

and exchanging limbs.

及對換肢體

Now how many people do you think

現在你猜各個形態中

there are in each state?

分別有多少人呢？

Half and half, right?

一半一半？

No, well, maybe.

不...唔...或許

It depends.

看情況而論

It could be 50/50,

可以是 50/50

but it could be 60/40

但又可以是 60/40

or 15/85,

或者 15/85

or anything.

或者任何比例

We chemists have to get our little, gloved hands dirty

我們化學家需要弄污戴了手套的一雙小手

- ah, well, we're in a lab so not really dirty -

——呀，唔，我們在實驗室中，不會太骯髒的——

to figure out what the actual distribution

去找出分子的

of molecules is.

實際數目分佈

Even though each of limb transfers

雖然每一次的肢體轉移

is a pretty dramatic event for the people involved,

對涉及的人來說都是十分戲劇性的

if we zoom out,

如果我們將鏡頭拉遠

we see population numbers that don't change.

我們會看見人口數目其實沒有改變

We call this nirvana equilibrium,

我們稱之為涅槃平衡

and it doesn't just happen with chemical reactions.

而且這並不只發生在化學反應上

Things like gene pools

如基因庫

and highway traffic show the same pattern.

及高速公路都有同樣的情形

It looks pretty still from 30,000 feet,

從高空三萬呎看下去似是靜止

but there is lots of crazy stuff

但其實有很多瘋狂的事情

happening on the ground,

在地上發生

you just need to zoom in to see it.

需要你將鏡頭拉近看看