What is fire anyway?
火究竟是什么？
Well, fire is rapid oxidization.
火，即燃烧，是快速的氧化反应。
It's the energy released whenoxygen atoms start combining with carbon and hydrogenatoms.
氧化反应是氧原子和碳原子或氢原子结合释放能量。
Exactly, so without oxygen in the air, nothing burns.
没错，那么如果空气中没有氧气，就不会有燃烧。
You mean there was a time before oxygen?
你是说氧气还不存在的那个时代？
Yes. By roughly a billion years ago, simple forms of plant life called “eukaryotes” were already busilyconverting carbon dioxide into oxygen gas, just like their descendants do today.
对，大约十亿年前，和眼下的植物一样，地球上最原始的植物祖先“真核细胞”忙碌地将二氧化碳转变成氧气。
But before thisprocess had been in place for a while, there wasn't enough oxygen in the air for things to burn.
但是，在此转化之前的一段时间，空气中没有足够的氧气可以燃烧东西。
In fact, of all the planets, moons, asteroids, and comets in our solar system, the only place whereyou can actually start a fire is right here on earth.
而事实上，整个太阳系的所有的行星、月球、小行星、彗星，你会发现唯一可以生活的地方只有地球。
And even here, it's only been possible relativelyrecently.
而就算是地球，也是不久以前才可行。
