In Fullmetal Alchemist, alchemy is the science of understanding the structure of matter, breaking it down, then reconstructing it as something else. It can even make gold from led, but alchemy is a science. In order for something to work, you must give something up of equal value.

[B]Exchanges in the anime.[/B]

Alphonse: Whole body. Edward: Left leg. Regain: Humonculi of there mother.

Izumi: Digestive organs. Regain: Humonculi of there baby.