It is just another day in Harvard Square.

Dizzy Gillespie, the legendary jazz trumpeter with the bullfrog cheeks, passes through on his way to the Harvard Coop for a book-signing.

A woman with a nosering sits outside Au Bon Pain and lectures a shaggy homeless man: "I mean, if you look at it the way the yogis or the Zen Buddhists do..."

A world-famous biologist stops at Cambridge Trust Co. to get cash from the automated teller.

A fat kid with his arm in a cast loses at chess.

Go ahead, stare at them. Everybody does it here. In fact, it's one of the best activities Cambridge has to offer. If, like most of us, you have a healthy curiosity about the behavior of your fellow human beings, you'll find Harvard Square a people-watcher's paradise, a sort of wildlife preserve of our own species.

One of the best things about the science of people-watching is that it requires no previous expertise. There are certain types you'll learn to recognize almost immediately.

Easiest of all, perhaps, are the tourists. No matter what their age or nationality, they all walk three times more slowly than everyone else. And they all carry shopping bags from the Coop.

They are retirees, some of them, queueing politely at the information kiosk by the escalator to the T, the husbands and wives in identical plaid pants and name tags that say "Elderhostel." The highlight of their visit will be a tour of Cambridge in the Old Town Trolley, which is actually a bus.

Some of the tourists are here with a more serious mission: they are teenagers with their parents on the ritual trip to visit colleges. Yesterday they stopped at Wesleyan and Yale; tomorrow, there are interviews scheduled at Amherst and Williams. The parents always look nervous; the kids either look nervous or bored.

Japanese sightseers, on the other hand, greet Harvard's scenic attractions with unrepressed delight. On a campus tour, they point excitedly at almost everything that catches their attention: At the buildings in the Yard. At the trees in the Yard. At the squirrels in the Yard. At you. This flurry of outstretched fingers is accompanied by the busy clicking of camera shutters.

A tourist will be gone the day after tomorrow. But many of the people you see in the Square will seem as permanent a part of it as the University itself.

The typical graduate student, for instance.

It's a hot day in mid-September, but he's wearing the same woolen sweater that he had on last February. The garment has gotten shabbier since then, but its owner has gotten no closer to finishing his dissertation.

You'll often see such would-be Ph.Ds crouched over books with titles like Proust and the Politics of Body, their facial muscules taut with what seems to be intense intellectual eagerness. Don't let that lean and hungry look fool you, though--it's due as much to physical as to metaphysical causes. In other words, most grad students don't get enough to eat.