All necks crane upward to check the height of what was once Chicago's tallest structure.
It's the 16-story Manhattan Building in the South Loop, designed and built in 1890 by William LeBaron Jenney and promptly nicknamed ''Hercules'' for its then-colossal height and size. Not exactly a giant, compared with such soaring modern works of steel as the nearby Sears Tower and John Hancock Center.
Nonetheless, it is a tower worthy of attention, as Doris Lohner, a Chicago Architecture Foundation (CAF) tour guide, tries to tell us.
''Now what do you suppose Jenney was trying to do in this space?'' she asks, sifting the answers before seizing on one.
''Right, he was trying to give the building as much light and air as he could by those bay windows.''
The day is magnificent, and we are off and running to inspect yet another Jenney landmark, the Leiter II Building, a block or two away on State Street. Suddenly, Mrs. Lohner stops in her tracks.
''Let's stand here because I want you to see the full sweep of the building, '' she says. ''What's different about it? What about ornamentation and rhythm? Do you see any?'' Decked out in sturdy brown walking shoes for the occasion, this retired junior high school history and language teacher admits she keeps her tours on the move.
''If I'm going to cover the ground in two hours, I have to walk fast,'' she explains.
Mrs. Lohner is one of about 550 Chicagoans who, over the last 12 years, have taken an intensive 10-week course offered by the CAF to encourage greater appreciation of the city's rich architectural heritage.
To spread the resulting light and enthusiasm, graduates lead more than 30 CAF tours - ranging from a bike tour of homes designed by Frank Lloyd Wright in suburban Oak Park and the standard Chicago Loop walk to strolls past the monuments of Graceland Cemetery and special tours in historic trains along the city's elevated tracks.
While the ranks of active tour guides have become winnowed to about half the number of course graduates over the years, most are amateurs like Mrs. Lohner - leading busy lives in other career fields, but enjoying architecture enough to give tours for no pay.
While CAF is grateful to have volunteers, it won't accept just anyone who wants to take the course.
''I had to wait eight months after applying,'' notes Wendy Kilcollin, a recent graduate and an architect who recently moved to Chicago from Washington, D.C. ''They're pretty selective.''
Last year CAF graduated about 60 volunteers, more than it could usefully assign to tours. As a result, this year's class was limited to less than half that number.
To qualify, candidates must be interviewed and sign a contract, pledging to attend all course sessions (usually 10 consecutive Mondays or Saturdays during the winter months), to complete all the homework and tests, and to give 40 hours of tours in the year after they finish. Absences are not lightly tolerated.
''In some ways it's a bit overregulated, but we care a great deal about quality,'' says Jethro Hurt, an education consultant to the CAF, who has taught the course over the last five years. ''It's not insulting training; it's challenging,'' he added. ''It's amazing how many people are talked down to by TV programs, and we don't want any part of that.''
''It's really a university-level course,'' offers Shuli Graham, a former New Yorker who has a master's degree in computer programming and who recently completed the CAF program. ''I'd say I put as much work in as I did at Columbia Graduate School.''
''It's like pledging a fraternity,'' says Al Vagner, a buyer for Sears, Roebuck who got interested in Chicago's architecture when he began reading books on local history a few years ago. ''You look back on it fondly, but you wouldn't exactly want to do it again.''
Mr. Hurt, by his own admission, is a hard taskmaster. It is with obvious pleasure that he admits that his students gave him the ''Idi Amin'' award a few years ago for having worked them so hard.
''He writes superb test questions that make you think,'' recalls Joy Hebert, an education consultant who recently completed her second CAF course to guide tours through the newly acquired Widow Clark House. ''I wrote for three solid hours on the final exam. It wasn't facts and figures. It was 'Consider the significance of balloon-frame construction in Chicago history' or 'Describe the steps in the construction of a building.' ''
The Chicago Architecture Foundation and its guide program emerged in the mid- 1960s when a printing company, which had been operating out of Glessner House just south of the Loop, moved to Pittsburgh. The granite home, which resembles a fortress on the outside, has a handsome wood-paneled, 35-room interior and an inner courtyard. The house was designed by architect Henry Hobbs Richardson and built in 1886 for J. J. Glessner, the founder of the International Harvester Company.
Aware that the house was a treasure, a group of young people in varied professions decided to band together to try to raise the money to save it.
''As the story goes, they decided over pizza in the spirit of Mickey Rooney and Judy Garland that they would form a foundation,'' says Hurt.
By appealing to a handful of corporations, they were able to raise enough to buy the property. They rolled up their sleeves and cleaned out much of the debris in the house themselves. They scraped off paint to get back to the original oak paneling. One of them was Jeanette Fields, who, according to Hurt, was serving the house as a ''one-man army'' - janitor, secretary, treasurer, and more.
Visitors, hearing about the venture, would occasionally knock on the door and be given an informal tour.
In 1970 Mrs. Fields (who happens to live in a Frank Lloyd Wright house) and a few other civic leaders, including current CAF board member Marian Despres, raised some funds from the Illinois Arts Council and officially launched the docent training course.
''We got good people to teach right from the beginning,'' recalls Mrs. Despres, a woman with real gusto when it comes to describing the importance of the Windy City in the history of American architecture.
''Architecturally, this is the most important city in the country; there's no question about it,'' she insists. ''This is really where the method of building the steel structure of the skyscraper was developed. Chicago is the cradle of modern architecture.''
The great Chicago fire of 1871, though a tragedy, is generally credited with paving the way for this city's architectural greatness. Talented and innovative engineers and architects, sensing opportunity, rushed in to fill the void.
The 19th-century masterpieces they left behind include the Monadnock Block, now the CAF headquarters. It is still the tallest commercial building supported by outside stone walls; interior steel framing supports the walls of taller structures. Others are Louis Sullivan's highly decorated Carson Pirie Scott & Co. and Auditorium Building, and the Rookery, with its controversial revamped Wright interior.
There are those here who say the average Chicagoan knows more about the variety and richness of this city's architectural heritage - and is more likely to tell visitors about it - than residents of most other cities, including New York.
And there is no discounting the fact that a lively debate goes on in this city as to where the best view of the incomparable Chicago skyline is to be found.
Some say the only place is a sailboat several hundred feet out from shore in Lake Michigan. But Chicago Tribune architecture critic Paul Gapp suggests that the prime spot at night is 2,500 blocks south of the Loop on the Dan Ryan Expressway. There, he tells readers, one can look up at ''a thousand points of light'' in windows of the city's ''giants.''
''It is a kind of poetry,'' he writes. ''No one can see it and not be moved.''
While the CAF's professionally trained guides are generally advised to keep their opinions on the subjective aesthetics of architecture to themselves, they frequently engage in lively arguments with one another over whose work they do or don't like and why. Most docents, for instance, know that Mr. Vagner considers Chicago ''over-Miesed.''
''I don't like Mies van der Rohe buildings in abundance,'' he says of Chicago's most famous architect.
''Chicago has Mies, son of Mies, and return of Mies. I saw one of his buildings once in Toronto, and it was absolutely magnificent, but it was alone.
''Most CAF docents are willing to give so much time to this particular cause, they say, because of the rewards in the learning itself and in the response they get from tour-takers.
For some, such as Mrs. Lohner, who takes the Socratic approach of asking questions on her tours, architecture is one way to continue teaching.''I'm interested in getting people to think, getting the wheels to turn around,'' she says. ''You have to do something, whether it's tantalize, amuse, or question, to get their eyes to light up. Otherwise, they're just going to be in neutral and get nothing out of it. If I have responsive people on my tour, that's my reward. I come home exhilarated.''For some - such as Mary Ann Wencel, a mother of four who spent 15 years as an accountant - getting involved with CAF has opened a new career path. She is studying for a professional degree in architecture at the University of Illinois.
Ultimately the link between understanding buildings, appreciating them, and saving them from the wrecking ball is a close one. William Donnell, the speaker at last year's docent graduation, told the guides: ''You are the Green Berets in the effort to save remaining buildings. If people love these buildings, they will be preserved.
''Your charge is to help people appreciate them.''