Alchemy. Not turning lead into gold or anything, though people could do that. It was against the law of course, but it was possible. Mine was transmuting water, specifically to make it freeze.

[Freezing people, no big deal!!]

Everyone who's heard of alchemy here who's not from Amestris always assumes it's turning lead into gold, so I figured I'd clarify.

[...he's so awkward, sorry.]