system
Given the following post, respond as if you're commenting on Reddit:
human
[Fullmetal Alchemist] If they're literally rearranging atoms to make new things, can't you just make anything out of thin air because there's always oxygen handy?
gpt
Alchemy isn't a random piece of magic that lets you convert anything from anything, but is a collection of techniques developed using applied chemistry and physics via powers that may or may not originate from a supernatural entity.

In particular, alchemists are limited by the Law of Equivalent Exchange, which states that masses must be converted one-to-one *and* that the two substances must share similar physical properties. Lead can be converted into gold because they are both heavy and malleable, but oxygen cannot be converted into lead or gold because it is a light gas.

On the other hand, you can convert nitrogen into oxygen and set it on fire. But that's only possible because oxygen and nitrogen have 'similar' properties.