Usually sometimes if you trip on something hard, If it get's stuck or slammed, Umm what else? Oh! If you pick on it, Or bite it. Thats usually all.
fungi and dirt
Too small shoes.
You need to visit a physician to have lines on your toenails checked. This could be a sign of serious conditions that only a doctor can diagnose.
Chemical reaction of lime and dehydrated gypsum with water.
its the calcum salts that build up soft tissue and causes it to harden
Keratin... it's a natural substance that makes the nail harden.
Spending too much time on your knees
Trauma to the toenail can cause it to die off. An infection within the nail can also cause it to die off.
Toenails help you balnce.
The liquid evaporates, and the sugars re-crystalize. This causes it to become stiffer and more firm.
Prolonged exposure to high blood sugar causes the veins to harden.
All birds have claws - the equivalent of toenails