It's one of our most important tools, and holds a prominent place in many ancient philosophies and religions.

这是我们最重要的工具之一，在很多古哲学和古宗教中占有很显著的地位。

The ancient Greeks believed that fire–along with earth, water, and air–was one of the four essential elements that made up the world.

古希腊认为，火，土，水和气是组成这个世界的四个基本要素。

We now know that the world's a lot more complicated, with over a hundred elements of matter which can be combined in a tremendous variety of ways.

现在，我们知道世界的构成其实是更加复杂的，由上百种物质组成，而同时这些物质又可以以无数种不同的方式结合。

This might leave you wondering where fire fits in. What exactly is fire?

那这就让你感到奇怪了，火是从哪里来的，具体是什么东西呢？

Watching a flame dance through the air, you might conclude that fire's a gas, like oxygen or carbon dioxide.

看到空气中的跳动的火苗，你可能会觉得火是一种气体，像氧气，二氧化碳那类的。

It's not. Fire can burn fuel that's a gas, or a liquid, or even a solid–as in the case of glowing charcoal.

其实不是。火可以使气体，液体甚至是固体燃料燃烧-比如燃烧的木炭。

But the fire itself isn't any of these things.

但是火本身并不是这些东西的任何一个。

In fact, fire isn't any thing at all.

实际上，火没有自身的形态。

It's not its own type of matter; it's something that matter can do.

它不是物质，但却起到一些物质的作用。

Fire is a chemical reaction.

火是一种化学反应。

A fire needs oxygen and some kind of fuel.

火是需要氧气和某种燃料共同作用的。

This fuel–whether it's candle wax, wood, or gasoline–usually contains big molecules that have carbon atoms inside them.

这些燃料不管是石碏、木头还是天然气通常都会含有大量的碳分子。

You can think of these molecules as little containers of energy.

你可以将这些分子想象成小小的储能体。

When they're allowed to combine with oxygen, this energy is released as heat and light.

当它们与氧气结合时，这些能量就会作为光和热释放。

Fire is a rapid chemical reaction known as oxidation.

火是一种快速的氧化化学反应。

Inside a fire, oxygen molecules break bigger molecules apart into carbon dioxide and water vapor.

在火中，氧分子将更大的分子氧化成二氧化碳和水蒸汽，火中的所有光和热都来自于含碳的大分子同氧气的结合。

All the heat and light of a fire comes from big, carbon-based molecules combining with oxygen.

火所有的热和光来源于碳分子和氧气结合的反应。

So what is fire? It's not the fuel or the oxygen or the heat or the light.

那么到底什么是火？它不是燃料，不是氧气，也不是光和热。

Fire is what happens between all these things. It's a chemical reaction.

火就是它们之间所产生的一种化学反应。