system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why protein is so important for muscle building

I understand that it's a basic fact that calories in > calories out will lead to increases in size in both muscle and fat. Why is so important, however, that protein (ideally) makes up so much of these calories? Thanks in advance.
gpt
Imagine a steam locomotive. It can burn coal or wood in its fire, and that gives it enough energy to drive around and pull trains.

Now imagine an old wooden caboose. We can build trains out of wood, or we can burn wood to power trains. Also, we can burn coal to power trains, but we can't build trains out of coal.

Proteins work like that in your body: You can build more muscles out of protein, or you can burn the proteins and use them for energy.

But just like how you can't build a caboose out of coal, you can't build muscle out of sugar.