When he was a little child, Andrew had a serious bad habit of sucking  his fingers all day long, especially when he had nothing interesting to do: when he was sleeping, when watching TV, when sitting on the sofa and so on. At the beginning, his parents didn't think it was a bad habit. Therefore, as time goes by, the boy couldn't give up his habit. Now, in his twenties, Andrew seems to have developed the bad habit into another one: he always bites his nails . He cannot stop biting his nails all the time. There're moments when he even doesn't know that he is doing it. He realized it was a problem and this worried him a lot. Some of his friends told him about this bad habit, and he stops for a while. But then, if he isn't busy doing something, he starts biting his nails again! In fact, there're a lot of bacteria  in your nails and they can cause some disease. Also, the nails will become ugly and hard to _ . The worst thing is that this bad habit will bring the mental illness . If people cannot correct it in time, it can affect man's mental health. In short, bad habits are very worrying, especially when the person with the habit is in public and cannot stop himself or herself doing it. Still, there must be ways to give up these bad habits and these people should try their best to stop doing them. When Andrew was a little boy, he had a bad habit of  _  .
biting his fingers biting his nails sucking his nails sucking his fingers