Why do some toddlers bite their fingernails?

Hello, I do not think this is specific to toddlers. I am 38, and I still, at times, bite my nails. In my family itself, there are number of people who bite their nails. Yes, they do not do it all the time, but they do at some point of time, because their nails are all chipped down till their skin. My youngest daughter has this habit. And I have noticed, she does it at the moments when she is thinking something. I point each time she does that and she stops too. She even promises not to do it next time, but doesn't get to realise when suddenly finger slips in her mouth. I think, it is just one of those habits that we get.