Why some population bites their nail?

Why some population bites their nail?
Some people usually bite nail when they feel bored or when they r surrounded by tension u can see a personage in our cricket troop biting his nails,it is due to restlessness even some in anger also we see biting their nail
i do not know
It's usually a nervous mannerism.
Response to stressful environment
i do cause its a bleak habit, and fruitless habits are firm to get rid of. I know it's gross, but when I bring back nervous, I guess it help.
I do. It's just pure need.
I also notice that I bite my nail when I am bored, impatient or waiting for someone or something.
Beacuse of Tensions.
It shows their mental instability.
1) Due to the nervousness.
2) If nail become to longer
It is same as some people touch their chin and some affianced in vibrate legs.
This reflects scarcity of concentration in the work they are occupied in.
Its indication of disterbing mind. when entity is thinking over much at that time they bites their nails.
dependence or their nervous just about something
because their parents doesnt gives them food to put away