Say two people are walking down the street, and they bump into each other.
当两个人走在街上，不小心撞到一起。
They'll just shake it off and walk on.
他们会立马分开，然后继续前行。
Sometimes that happens with molecules too.
这种情况有时也发生在分子身上。
They just bounce off each other, and that's that.
不过它们只是互相弹开而已。
But what if two people were to bump into each other, and during that collision,
但是如果两个人撞到一起，在这个过程中，
one person's arm got severed and reattached to the other person's face?
其中一个人的手断了，粘到了另一个人的脸上，情况会如何呢？
Now that sounds really weird, but it's similar to one of the many ways that molecules can react with each other.
这听起来很奇怪，但这与分子间发生反应众多方式之一非常相似。
Two molecules can join and become one. One can split apart and become two. Molecules can switch parts.
两个分子可以结合，变成一个。一个分子也可以分裂成两个。分子可以相互交换部件。
All these changes are chemical reactions, and we can see them happening around us.
所有这些变化都是化学反应，它们就发生在我们身边。
For example, when fireworks explode, or iron rusts, or milk goes bad, or people are born, grow old, die, and then decompose.
例如燃放烟花，铁生锈，牛奶变质，人的出生，变老，死亡，最后分解。
But chemical reactions don't just happen willy nilly! Everything has to be right.
但是化学反应不会平白无故的发生，所有条件都要具备。
First, the molecules have to hit each other in the right orientation.
首先，分子要以正确的方式互相碰撞。
And second, they have to hit each other hard enough, in other words, with enough energy.
其次，他们得相撞必须足够强，换言之，就是有足够的能量。
Now you're probably thinking that a reaction just happens in one direction and that's it.
现在你可能会想，化学反应只沿着一个方向发生。
Sometimes that's true. For example, things can't unburn or unexplode.
有时这是正确的。比如东西不能逆燃烧或是逆爆炸。
But most reactions can happen in both directions, forward and reverse.
但大部分的化学反应可以沿着两个方向发生，或正或逆。
There's no reason that our face-arm guy can't bump into armless girl, reattaching that arm back to its original socket.
脸上有手臂的男孩也不是不可能会再次撞到无臂女孩，从而把手臂接回原来的位置。
Now let's zoom out a bit. Now let's say that you've got a thousand people on the street,
现在，让我们拉远一下镜头。假设现在有一千人走在街上，
and all of them start with their limbs normally attached.
开始他们的四肢都在正常的位置上。

At the beginning, every collision is a chance for Person A to transfer an arm to Person B's face.
刚开始，他们每一次相撞，路人甲的手臂都有机会黏在路人乙的脸上。
And so at the beginning, more and more people end up with arms attached to their faces or arms missing.
所以一开始，会有越来越多人的手黏到别人的脸上或是失去了自己的手。
But as the number of people with arm-faces and missing arms grows, collisions between those people become more likely.
随着这些人数的增加，他们相撞的可能性也就随之增加。
And when they bump into each other, guess what?
那么当他们相撞，猜猜会发生什么事？
Normal-appendage people are reproduced.
他们又变回了四肢完好的正常人。
Now the number of limb transfers per second forward will start high and then fall,
这样说来，每秒肢体脱节的数目，从刚开始很多，逐渐减少，
and the number of limb transfers per second backward will start at zero and then rise.
同时每秒肢体回到原位的数目，会由零开始逐渐上升。
Eventually they'll meet, they'll be the same.
最后他们会不断趋近，数目相同。
And when that happens, the number of people in each state stops changing,
当这一情况发生的时候，每种形态的人的数目将停止改变，
even though people are still bumping into each other and exchanging limbs.
尽管他们仍在互相碰撞并交换肢体。
Now how many people do you think there are in each state?
现在你觉得，每种形态的人数会有多少呢？
Half and half, right? No, well, maybe. It depends.
一半一半对吧？不，嗯...也可能吧。这要依情况而定。
It could be 50/50, but it could be 60/40 or 15/85, or anything.
可能是一半一半，也可能是60/40，又或者15/85，或是任何比例。
We chemists have to get our little, gloved hands dirty -- ah, well, we're in a lab so not really dirty
我们化学家必须亲自动手，弄脏带的手套，哈哈，恩，我们的实验室并没有那么脏的，
to figure out what the actual distribution of molecules is.
去找出实际的分子比例。
Even though each of limb transfers is a pretty dramatic event for the people involved,
即使每一次肢体交换对参与者来说是非常神奇的事情，
if we zoom out, we see population numbers that don't change.
如果我们将镜头拉远，我们会发现人群数目不会发生改变。
We call this nirvana equilibrium, and it doesn't just happen with chemical reactions.
我们将这种重生称之为平衡，它不仅存在于化学反应中。
Things like gene pools and highway traffic show the same pattern.
其他比如基因库以及高速公路也有这种现象。
It looks pretty still from 30,000 feet, but there is lots of crazy stuff happening on the ground, you just need to zoom in to see it.
从高空三万英尺看下去好似静止，但其实有很多疯狂的东西在地面发生，你只需将镜头拉近即可发现。