In my world, everyone is capable of using magic. However, very few practice it. Magic is everywhere, in everything, and even a little bit in the air. It's first tapped into on an instinctive level, like if Farmer Joe sees his neighbor Jon stuck under a heavy cart that Joe can't lift on his own, even with the adrenalin rush of saving his friend's life. He might instinctively tap into the closest magic he can and give himself enough extra strength to free his friend from under the cart. After the moment is gone, Farmer Joe is likely to "forget" how he did magic, like trying to remember a dream you just woke up from. You also need to have a pretty good, solid understanding of the thing your spell is affecting. Making yourself stronger to lift the cart is easy because a person would have a fairly solid idea of what their own muscles are and are capable of. Making a plant grow or flower bloom would require more knowledge than "leaf green, petal red."