system
Given the following post, respond as if you're commenting on Reddit:
human
[Full metal alchemist brotherhood] Hoeinheim made this huge deal out of ed/al not transmutimg their actual mom and apparently it was something else, who pr what exactly did ed/al and Izumi transmute??? did they just create another life ?
gpt
you can make a body with common elements. That's what the boys did. 

The problem ends up being when you try to bring it to life. Life isn't just base matter. It's energy. that energy has to come from somewhere. In order to get the animating force, the energy,  the "soul", you have to open the gate. The boys didn't realize what this would mean, what price they would have to pay. They just wanted their mom back. 

The problem is, Mom was gone.  They didn't have her original body, they didn't have anything but themselves to draw her back... so they opened the gate to find her, and what their alchemy called out was not their mom - it was something ELSE. Maybe it was a soul not quite through with the reincarnation process. Maybe it was a demon or an Outsider. Maybe it was an angel, fallen from grace. Who knows? 

A homunculus is what happens when the things on the other side of the gate get into a body - either an artificial body made with alchemy, or an existing body, through the Philosopher's Stone. The boys did make life... but not well. What the boys made was... not a healthy human body. What was inside it was not their mother. They made a homunculus, and Mustang put it out of its misery before it had a chance to do more than crawl.