As you will be aware, your penis has already grown naturally once before, and that was during puberty, at which time you saw tremendous growth. The reason this growth took place is because your body was producing nutrients, whose role was to reach your penis through the bloodstream and help to form new growth cells. When you reached adulthood, your body no longer produced the nutrients, so your penis stopped growing. Now, if you are able to reactivate production of these nutrients, your penis can grow again naturally, in the same way that it did during your pubescent period.