Super Power/Mutation: The alteration of his basic molecular structure into water. It would give him the ability to deflect attacks, slip under doors or into cracks and basically have the same abilities of Sloth in the 2003 anime. Since it's more for the sake of disturbing Ed than giving him an ability I'm open to any other restrictions that might be suggested.
Restrictions: He wouldn't be able to destroy objects like she can, so it would be basically limited to the getting through small spaces, spying and dodging aspects of her ability. Clothes can not be transformed. I was thinking that the automail would be transformable, although I don't remember the first reason I had - the second was because it is connected to his body by the nerves, making it in some aspect part of him. He wouldn't be able to control it all the time or at all when it first started. I kind of imagined he'd learn about it by some imagined threat triggering the reaction in his brain to liquify in order to dodge the 'attack'. I didn't really see it being something that would ever tire him, mostly because I don't see him willingly using it long enough to get tired. But there's no reason he couldn't be 'stuck' for a bit, meaning the time he's changed and the effort to change back could drain his energy at a faster rate.
Also I got other opinions about it before posting and someone suggested that since he has the metal of the automail it would alter the composition of the liquid form, and not be pure water?
Reasoning: Irony, mostly. It wouldn't be an ability he'd enjoy or even want to take advantage of and it would remind him things he'd rather keep in the past. At the same time, with the logic that the gas could manifest something that weighs heavily on a character's mind, it is something that I doubt he would have ever gotten completely over, creating Sloth, killing Sloth - Sloth in general. So I thought it would be oddly fitting in that sense.
Any last questions? Could I be any more last minute? Okay, seriously - I don't think so... Unless some further explanation is needed from me. I don't pretend to think I make sense. I don't want to assume anyone's familiar with the canon I'm referencing so I was going to include links but I wasn't really happy with any of the sites I found explaining the ability, either.