Summary: Our minds are like a sponge. What we put in them gets absorbed and stored. If you are constantly fueling your mind with good and positive information, then those are the things that make up your thoughts. If you are constantly feeding your mind with junk, limiting beliefs, and negative content, then those are the things that make up your thoughts. When you realize this, you begin to guard your mind against that bad input, and instead consume as much good content as possible.
