Across town, the 50,000-seater Soldier Field, home of the Chicago Bears is being made ready for music.
"We have to remove the sod from the stage area," explains the lady from City Hall. Has Bill Wyman been trying to rejoin the band?
Stadium rock sprouts its own terminology. The same lady asserts that a security barrier will prevent younger fans from "rushing up to form mosh pits".
Away from the mosh pit, 54-year-old Jagger is with his personal trainer. Tonight he will dance, strut and gyrate the equivalent of five miles on stage, provoking the Chicago Tribune to run a depressing cover story, "Pecs and Diet and Rock and Roll" on the health and fitness secrets of ageing rockers.
Jagger runs and cycles; Aerosmith singer Steve Tyler has banned sugar, salt, wheat, yeast, fat, red meat and alcohol from his band's menus. Even the Grateful Dead, while publicly burning the Bacchanalian flame at both ends, were secretly calorie watching. Band member Bob Weir confesses: "In the Dead, you would find four or five of us in the gym on any given day."
Only, and inevitably, Keith Richards preserves some illusions. Asked how he prepares for a year-long world tour, he replies: "My workout? I play with the Rolling Stones."
For Richards and the others, being here in Chicago is a nostalgic experience. The Chicago bluesmen were their earliest and prevailing influences. When they recorded "It's All Over Now" at the city's Chess Studios in 1964, they hoped they would catch a glimpse of Muddy Waters. And they did. He carried their bags in before painting the walls of the studio. In those days bluesmen didn't give up the day job, and Waters, a painter-decorator, was no exception.
Whether out of sentiment or the need for just one more rehearsal, the band played a small Chicago blues club unannounced at the weekend. Among the audience was the American secretary of state George Ryan, aged 63, who did the group no favours remarking to the press afterwards that they were much closer to him in age than to anyone else in the club. The rumours of an unannounced gig did wonders for Chicago's blues and folk circuit. Every club sold out on Saturday night.
And so the Stones begin yet another comeback, their American tour virtually sold out, the two opening shows in Chicago alone playing to 100,000 people.
Band and well-heeled fans play a game of mutual self-delusion - it's all about music, back to basics blues rock, and a rejection of corporatism. The band's publicity machine trumpets their Chicago connection and the city glories in it, with the crinkled faces of Jagger and Co all over the front pages.
The truth is more prosaic. The tour starts in Chicago because all the shows are outdoors and the itinerary has been planned so that they can visit each city before the cold spell sets in. The ticket sales are indeed huge, but significantly corporate sponsorship for the first time is building a customised audience.
The tour sponsor Sprint, the telecommunications giant, is also acting as ticket broker with its customers getting first shot at some of the best seats, as high as 50 per cent of them in some venues. It is easy to foresee that this is the start of a new trend in selling tickets for major events.
To live to affluent old age, the Rolling Stones must now listen to telecommunications sponsors and meteorologists rather than blues-singing painter-decorators.