Alchemy is the science of understanding, breaking down, and recreating forms of matter into another form. Like... you can turn a chair into a little table with alchemy! But it has to be of equivalent exchange, that's really important to remember. You can't make a really big table out of a little chair... Well, you probably could but the material would be very thin and it would break easily... And it has to be similar, so you can't make water out of wood and things like that...