Considered normal in one situation and abnormal in another. for ex. if a person has the habit of checking twice, whether he has locked the door properly, his behavior will be considered normal, but if he repeats this activity again and again, then his same behavior will be considered as abnormal. this is an example of obsessive compulsive disorder. another ex. if someone has a fear from water, this can be considered as normal, but if someone has such an intense fear of water that he starts crying even at the sight of water, then this will be considered as abnormal. this is an example of hydrophobia. thus any behavior which becomes intense with time without any rational reason may lead to mental disorder.