Bishop H. Burke Petersen further explained: "When we were born in the earth, our minds and thoughts were clean and sweet and pure—unpolluted by the harmful impurities that are around us that become a part of the experiences of this life. In our infancy our minds are free from unrighteous and unwholesome thoughts. We are innocent and untouched by most of the harmful effects and influences of Satan.
"Our mind, which is like a tremendous reservoir itself, is capable of taking in whatever it may be fed—good and bad, trash and garbage, as well as righteous thoughts and experiences. As we go through life, we may be exposed to stories, pictures, books, jokes, and language that are filthy and vulgar, or to television shows and movies that are not right for us to see or hear. Our mind will take it all in. It has a capacity to store whatever we will give it. Unfortunately, what our mind takes in, it keeps—sometimes forever. It's a long, long process to cleanse a mind that has been polluted by unclean thoughts." (H. Burke Peterson, "Purify Our Minds and Spirits," Ensign, Nov. 1980, 37)
The brain is immensely efficient, able to process large amounts of information in a short time. While the brain has only a limited capacity for conscious processing, the brain receives a lot more information than we are aware. This is why we need to surround ourselves with only those things that uplift. However, the brain cannot actively process very much at once. In other words, we can only really think of one thing at a time, even if we can jump back and forth between ideas rapidly. This may be one reason why we are told to pray always–so we don't have time to let our minds wander to places they shouldn't go.
Elder Maxwell taught, "The human mind is remarkably retentive. We must be careful of what we allow in our mind, for it will be there for a long time, reasserting itself at those very times when we may be most vulnerable. Just as harmful chemicals heedlessly dumped in a vacant lot can later prove lethal, so toxic thoughts and the mulching of the wrong memories in the vacant corner of the mind also take their toll" (Elder Maxwell, The Neal A. Maxwell Quote Book, p.346).