The good man brings good things out of the good stored up in his heart, and the evil man brings evil things out of the evil stored up in his heart. For out of the overflow of his heart his mouth speaks.
Notice that it is out of the overflow of the heart, the mouth speaks. So a person could store up bitternesses, bad thoughts about someone, repressed anger, but when the heart becomes full, these things will spill out of our mouths.
How do we keep this from happening? The answer is in the same verse – we store up good in our hearts. I looked it up in the Greek, and it is exactly as translated;” to store up”, but the definition is qualified by saying “store up for a special need.” The man who stores up good things in his heart is the one who will speak good things in the time of need out of the overflow of his heart. This is how I view personal Bible Study. We are storing up the good ” things”, the truths of God, that we might speak them to those in need, both sinner and saint.