What is your favorite proven or unproven myth?

NinjaMom • "In the end we will remember not the words of our enemies, but the silence of our friends" - Dr. Martin Luther King, Jr.
I thought this might be an interesting conversation. What is a myth - proven or unproven - that you believe in? Whether you've researched its validity or not, or you believe in it regardless and it guides certain things that you do, or traditions (or habits) you uphold in your life, what's something that you hold dear to you? 🙂 
Mine is Karma. I believe that everything we do balances out in some way, shape or form. So, if we do something nice for someone else, karma will allow good things to come your way. Likewise, if we do something cruel to someone else, someone will do something cruel to us. Not sure if that makes sense or not, but it's one thing I believe in wholeheartedly.
Please remember, these are personal beliefs and feelings.