Energy is not easy to define.
能量不容易被定义。
Things have energy, but you can't hold a bushel of energy in your hands.
虽然所有的物质都带有能量，但是没有人可以把能量握在手中。
You can see what it does, but you can't see it directly.
你虽然能够感知能量的作用，但是不能直接看到它。
There are different types of energy, but the differences between them are manifested only in how they make stuff behave.
能量分为不同的种类，各种形式的能量的不同之处表现在它们对其它物体的影响上。
We do know that the total amount of all the different types of energy in the universe is always the same.
我们知道，宇宙中的各种能量的总和是守恒的。
And, for chemists, two important types of energy are chemical potential energy and kinetic energy.
对于化学家们来说，有两种重要的能量，它们分别为势能和动能。
Potential energy is energy waiting to happen.
势能是一种等待发生的能量。
Think of a stretched rubber band.
比如说一根拉长的橡皮筋。
If you cut it, all that potential energy gets converted to kinetic energy, which is registered by you as pain.
如果你剪断它，它所带有的势能便转换为动能，弹到你身上让你有痛感。
Like a stretched rubber band, chemical bonds also store energy, and when those bonds are broken,
像拉长的橡皮筋一样，化学键也同样储存着能量，当化学键断掉时，
that potential energy gets converted to other types of energy, like heat or light, or gets used to make different bonds.
它所带有的势能便转换成其它形式的能量，比如热能和光能，或者是被用来形成另外的化学键。
Kinetic energy is the energy of motion, and molecules are always moving.
动能则是跟物体运动有关的能量，分子都是在不断的运动着。
They're not necessarily going somewhere, though they could be,
它们不一定都在移动，当然这也是可能的，
but they are vibrating, stretching, bending, and/or spinning.
但是它们是不断的在震动，拉伸，弯曲或者旋转。
Take methane, which is four hydrogens attached to a central carbon, as an example.
例如甲烷分子，它是由4个氢原子围绕着一个碳原子构成的。
Drawn on paper, it's just a still tetrahedron.
画在纸上，它看起来像一个静态的四面体。
But in real life, it's a jiggling mess.
但实质上，构成分子的原子是不停运动的。
The kinetic energy of molecules is exactly the same type of energy as the energy you have when you're moving around,
分子的动能跟人们在活动时所带有的能量是同一个概念，
except that you can be still and molecules can't.
不同的是，你可以保持静止，但分子不能。
If you suck the kinetic energy out of a group of molecules, they'll move less, but they'll never fully stop.
如果从一组分子中吸取它们的动能，它们将会减少运动，但不会完全停止。
Now, in any group of molecules, some will have more kinetic energy than others.
在任何一组分子中，一些分子带有的动能会比另一些多。

And if we calculate the average kinetic energy of the group,
如果我们计算整组分子的平均动能，
we'd have a number mathematically related to temperature.
我们将会得到一个与温度成比例的数值。
So, the more kinetic energy a group of molecules has, the higher its temperature.
所以，一组分子的温度越高，也就代表着它们的动能越大。
And that means that on a hot day, the molecules in the air around you
这意味着，天热的时候，在你附近空气中的分子
are spinning, stretching, bending, and generally shooting around much faster than on a cold day.
会比起天冷的时候旋转，拉伸，弯曲以及移动得更快。
Now, hot and cold, by the way, are relative terms.
不过热和冷也是相对的概念。
They're always used to compare one thing to something else.
我们总是用其中一个来和另一个做对比。
So, on that hot summer day, the air molecules have more kinetic energy than the molecules in your skin.
在一个炎热的夏天，空气中的分子比起你皮肤中的分子带有更多的动能。
So, when those air molecules crash into you,
当空气中的分子撞到你时，
they transfer some of their energy to the molecules in your skin, and you feel that as heat.
它们把一部分能量传递给你皮肤中的分子，然后你就感到热。
On a cold day, the air molecules have less kinetic energy than the molecules in your skin,
在天冷时，空气中的分子比起你皮肤中的分子带有更少的动能，
so when you crash into those air molecules,
当你撞到空气中的分子时，
you actually transfer some of your kinetic energy to them, and you feel that as cold.
你把一部分能量传递给它们，然后你便觉得冷。
You can trace the path of energy around you. Try it at your next cookout.
你可以观察自己身边的能量转换。比如下次野餐烧烤。
You burn charcoal and the release of that chemical potential energy shows up as extreme heat and light.
当你烧炭时，它们所释放出的化学势能表现为高温和强光。
The heat then makes the molecules of your burgers, your hot dogs, or your vegetables vibrate
这个热量会使汉堡，热狗，蔬菜等的分子震动，
until their own bonds break and new chemical structures are formed.
直到它们的化学键断裂，然后形成新的化学结构。
Too much heat and you have a charred mess; just enough and you have dinner.
如果过度加热，食物会变成焦炭；而适度的加热便给你带来可口的晚餐。
Once in your body, the food molecules in your delicious, or charred, dinner get broken down,
当进入你的身体后，你的可口（或烧焦了）的晚餐中的食物的分子将被拆开，
and the energy released is used to either keep you alive right now or it's stored for later in different molecules.
随之释放能量，这些能量要么被用来维持你现在的身体活动，要么被储存在其它分子中以后使用。
As night falls, the hot summer air cools and the flow of energy into you slows.
到晚上，空气渐渐变凉，而传递给你的能量也渐渐减少。
Then, as the air reaches your skin temperature, for the briefest of moments, the flow stops.
当空气的温度与你皮肤的温度相等时，在这一刹那，能量的流动停止了。
And then it starts up again in the opposite direction
然后反方向的能量传递重新开始，
as energy leaves the warmer surface of your skin to return to the universe around you,
能量将从相对高温的皮肤表面流向你周围的宇宙，
that energy, neither created nor destroyed, but ever shape-shifting, the chameleon phoenix of our physical world.
能量既没有被创造，也没有被消灭，只是不停地在转换形式，它称得上是物质世界的变色凤凰。