"Obsessive-compulsive" behaviors

I wonder when a behavior qualifies as "obsessive-compulsive." At what point does it stop being normal and become a disorder? According to readings I've done in my psychology classes, a behavior is not defined as being obsessive-compulsive unless it interferes with one's daily life. So it must be the type of thing somebody has to do a great number of times each day, because if it's only a few times, it really can't be said to be impairing one's functioning. My friend, for example, is convinced she has a little bit of OCD because she "has" to check the knobs on her gas stove at least three times every night before going to sleep. Even if they're clearly turned to the "off" position, she still has to touch them and make sure they're really off. What's more is that if anyone touches them after her, she has to touch them again--even if the knobs are still obviously in the off position. I tested this one night before leaving her house. She went through her routine, checking each of the four knobs three times. I then touched one, without turning it. She immediately complained, "Stop! Now I have to do it all over again." And she proceeded to check each one again. I touched the knob again. This time, she hit me, then checked and touched each knob again. It was pretty funny, because I know she doesn't have OCD. It doesn't significantly affect her life. She doesn't waste more than a few seconds a night on her paranoia that her eyes are deceiving her, the knobs are actually "on," and the house will soon blow up. I think if her behavior qualified as OCD, a lot more people would have this disorder. I myself engage in somewhat compulsive behaviors sometimes. I check the lock on the door to my house usually about three times a night, turning the deadbolt and trying to pull the door open with the latch (it's an old door and thus knobless) to make sure it's secure. But I wouldn't call this OCD. Because I don't spend much time doing it, unless you added up all the seconds, in which case over the course of a hundred years I might lose a day. Okay, I just calculated it. Four days over a hundred years. Approximately. If I spend ten extra seconds a night checking the door. It's worth it to me to feel safe, in today's world. Please feel free to comment and share your "compulsive" behaviors with me!