It is enamel. Like hair and nails, this hard substance is made by epithelial cells. Once it was thought to be as inert as rock; now it is known to exchange its substance with the body fluids ...

"At the beginning in 2002, I wouldn't even let my own friends invest as I didn ... "SpaceX is alive by the skin of its teeth, and so is Tesla - if things had just gone a little differently ...