Favorite quote or phrase?

What is a phrase that has stuck with you since the first time you heard it? Something that is always in the back of your mind in your daily activities and interactions with other people. Perhaps it is a small bit of wisdom someone took the time to impart on your mind as a youngster, or a favorite quote that you read or heard somewhere. I'll lead off with mine, given to me by my recently deceased father when I was about 12 years old. I'm sure it wasn't his originally, but hearing it from Dad really made it stick with me. He said to me, after I got into a tussle with someone who was harassing a friend of mine, "Son, never argue with an idiot, they'll just drag you down to their level, and beat you with experience". I admit, I sometimes have a short fuse to a large temper when it comes to what I view as stupidity, and at times it gets me in trouble, but I try to remember this phrase when I see that there is just no changing someone's mind about something, no matter how much evidence you may have to the contrary of their beliefs. It enables me to chuckle, and let things go at times when I have an issue keeping my mouth shut. So, what's a phrase or quote that you carry with you or that has had an impact on your life at some point?