Fullmetal Alchemist pushed Edward and Alphonse through the test of their own brotherhood.

Fullmetal Alchemist is a story about two brothers who know what it is to have each other's backs when all the world does is demand. Hiromu Arakawa, the mangaka behind the series, grew up understanding the corresponding exchange. According to Newton's third law of motion, for every action there is an equal and opposite reaction.

The laws of equivalent exchange also apply to a large extent in alchemy. It's all about the give and take of nature. Alchemy is about maintaining balance in the world. To combine nature and science into one big experiment and create something completely new from it. It's all about trial and error, but most importantly it's about respect and sacrifice. For two brothers who were ready to sacrifice their own selves for their mother, alchemy was a dark art.

In a theory suggested by a user on Reddit, Fullmetal Alchemist tests Edward and Alphonse's sacrifices, not just in the way alchemy takes from them but in the way they can give to it. In the need to get their mother back, the Elric brothers respectfully sacrifice their leg and body. The former, however, gains an astonishing knowledge of alchemy in the process. At the same time, he also sacrifices his arm to bring back Alphonse's soul but instead in body armor.