Teach them diligently — Heb. whet, or sharpen them, so as they may pierce deep into their hearts. This metaphor signifies the manner of instructing them, that it is to be done diligently, earnestly, frequently, discreetly.
Related Commentaries of Deuteronomy 6:7
Deuteronomy 6:7
7 And thou shalt teach them diligently unto thy children, and shalt talk of them when thou sittest in thine house, and when thou walkest by the way, and when thou liest down, and when thou risest up.