Problem:

From what I've understood from what I've read (which might be wrong), enamel rods extend away from the dentin in our teeth. As young people grow, the rods are thickened.

Question: What are the technical limitations and/or problems/challenges of trying to extend/grow the rods/crystals while the teeth are in the mouth?

One issue I guess could be that the enamel is porous, and so chemicals could affect the other parts of the teeth.

But one example of trying to improve the quality of tooth enamel is Novamin, which some manufacturers have claimed creates a glass on the tooth. But I don't understand what that means.

Could someone explain this to me?