Is this a fact or just a theory?

Well, someone told me that we don't use our whole brain on a day to day basis. They said that right handed people only use their right side of their brains and if they learned to write with their left hand they could learn to use the left side of their brain and vice versa for left handed people. I honestly think it's ridiculous mean I know we don't use all of our brain capacity but I don't believe her theory.