The mental institution just got a new shrink...

...and on his first day he is shown around the hospital and introduced to the staff and patients.

As he comes to the activity room, there is a group of patients sitting in a circle facing one another. One of them says: "Number 31!" And the others start laughing. He notices that one of the patients, a young man, laughs much more than the others.

Then another patient chimes in: "Number 107!" And everyone's having a good laugh again. However, the young man is so amused that he's slapping his thigh and laughing much more than the others. The new shrink starts wondering what is going on.

Once again, another patient says: "Number 10!" The patients all start laughing again. By now, the young man is almost keeling over from laughter. And the shrink is now so curious that he goes up to one of the patients and discreetly asks him what they are doing?

"We're telling jokes, Doc," the patient whispers back.

"Oh, but is seems you're only mentioning random numbers."

"Well you see, we have been here a long time and we've all heard the same jokes over and over, so we decided to save us some time by just numbering them all. So when someone says a number, the other ones remember which joke he is telling, and we all have a good laugh. It's very convenient."

"Oh, I see" says the doctor hesitantly, "but what about that young fellow over there? Why is he laughing so much more than the others?"

"Well, he's new here, you see, so he has never heard any of them before."