The Serenity Prayer goes, "God grant us the serenity to accept the things we cannot change, courage to change the things we can, and wisdom to know the difference."
Although the ending, "wisdom to know the difference", sounds like a punchline, the prayer is no joke. It is part of the most serious organized program of spiritual and religious development known to modern psychology (see Twelve Steps).
In life having the wisdom to know what we can change and what we cannot change is frequently the hardest part of an endeavour. How long do we keep trying to change something that resists change? When do we switch and look for something to do that is productive?