People use pieces of iron to sharpen iron knives. In the same way, people learn from other people, making each other sharp.
Iron sharpeneth iron; So a man sharpeneth the countenance of his friend.
Iron sharpens iron; so a man sharpens the countenance of his friend [to show rage or worthy purpose].
As iron sharpens iron, so one person sharpens a friend.
Iron sharpens iron, and a man sharpens the countenance of his friend.
Iron sharpeneth iron, so a man sharpeneth the countenance of his friend.