[Despite her reassurances, he still feels a cold pit of dread as he reaches up and removes his helmet to reveal his armour is completely hollow within. He bends down slightly so she can see inside, to the seal drawn in blood on the back of his neck.]

I am human, I am-- it's just...

[He replaces his helmet to explain.]

In alchemy there's a principle that everything else is built on, that's equivalent exchange. It's the rule that you can't take more than you give, what you create has to be of equal value to the components. That's why there are-- there are things it's forbidden for an alchemist to do. One of those is human transmutation, creating life.

But my brother and I, we thought we could do it anyway. We thought maybe it wouldn't matter if we broke that rule just the once. We didn't want to do anything terrible, we just wanted to see our Mom smile again after she died. So we tried-- we had all the ingredients that make up a human body, but we couldn't pay the price for her soul.

[His voice grows softer, slight break in the words.]

The transmutation rebounded on us and took the price that way. Brother lost his leg, and I lost my entire body. I would have died, but Ed gave up his arm to bind my soul to this suit of armour in order to save my life.