We often use the analogy of someone learning to ride a bike or drive a car for learning a fl. Do you just begin riding and driving or do you first learn the mechanics of the bike/car? Clearly, you cannot learn to ride/drive by studying how bikes/cars work; you have to actually ride/drive one.
Another analogy would be playing basketball/soccer: you can learn to play by getting out and playing or by studying the angles, force, speed, and distance formulas. The latter will not work and is analogous to studying how a language works, i.e. its phonology, grammar, syntax, etc.
My analogy limps badly b/c language is not a physical skill like gauging the angles, speeds, forces, etc. in soccer/basketball but is rather an ability. Nevertheless, it might help some skeptics get the point, something I grow more doubtful of with every passing day.