Matthew Henry and continuators on Proverbs 15:2

2 The tongue of the wise useth knowledge aright: but the mouth of fools poureth out foolishness.

Note, 1. A good heart by the tongue becomes very useful. He that has knowledge is not only to enjoy it, for his own entertainment, but to use it, to use it aright, for the edification of others; and it is the tongue that must make use of it in pious profitable discourse, in giving suitable and seasonable instructions, counsels, and comforts, with all possible expressions of humility and love, and then knowledge is used aright; and to him that has, and thus uses what he has, more shall be given. 2. A wicked heart by the tongue becomes very hurtful; for the mouth of fools belches out foolishness, which is very offensive; and the corrupt communication which proceeds from an evil treasure within (the filthiness, and foolish talking, and jesting) corrupts the good manners of some and debauches them, and grieves the good hearts of others and disturbs them.