Question

How tooth decay occur? Give applications.

Answer 1

For tooth decay to occur three things need to be present. 
1) the tooth (of course ) which is what is going to get decayed.
2) a bacteria that brings abt the changes that will lead to decay. 
3) a stimulant (carbohydrate/sugar) that will give the bacteria what it needs to cause the change. 

When any kind of carbohydrate is going to stick on to your tooth surfaces (thus most of the blame is on chocolates jams jellies biscuits pastries and sugar syrups) , the bacteria starts to ferment it. Which is a fancy way of saying that it starts breaking down the carbohydrate to its smaller products and this process releases a kind of acid (lactic acid) . 
The acid causes the structure of the tooth enamel to weaken and finally break down! And this, is decay! 

Time is also a factor here. The longer the carbohydrate remains stuck to the tooth surface , more the time for the bacteria to keep working on it and releasing acid. So brushing after every meal, especially after eating sticky carbohydrates is a great habit!