This seems like such an empowering way to look at learning, and one I think many of us are prone to forget. Even masters don't just arrive at their talent, they too have to work at it, over and over, day after day.
As with all analogies it should not be taken too far.
> practice of a specific mental skill has little influence on other mental skills
Care to elaborate on this? I'm trying to imagine what you mean here.
Instead, maybe emphasize that the brain is similar to a muscle. Speaking metaphorically could be confusing to some.
I disagree. Take reading, for example. Does reading not make you a better writer?