"Why did you put the alligator in Underground?" asks a little girl who has finally reached the head of the autographing line and is face to face with David Macaulay, author and illustrator of Underground and seven other sophisticated children's books.

"Well, actually," says Macaulay, who was born in England but moved to the United States when he was 11, "there's a story that in New York people brought baby alligators home from Florida as pets. Then they got tired of them and flushed them down the toilet. Some of these alligators -- it may have only been one -- actually survived in the sewer system."

At the Cheshire Cat Book Store most Saturdays and sometimes in between, kids get to put questions about the books they read directly to the people who wrote them. And the questions, like the kids with armloads of books to be autographed, keep coming. "What underground is it?" "It's bits and pieces of everywhere," answers Macaulay."It's London in terms of the subway, but I also poked into a lot of manholes in Boston." "About the research, how do you research it?" wonders a girl. "I go to the library and start there," answers Macaulay. "I read everything I can get my fingers on, then I go visit the place."

For Pyramid research took him to Egypt. For Cathedral , he went to France. City took him to Italy. But for his latest book, Unbuilding , Macaulay had only to commute from his home in Providence to New York City. Unbuilding is a story about dismantling the Empire State Building to ship it to its new owners -- some oil-rich Arabs. It is sort of, but not entirely, a spoof.

"It's hard to be bizarre these days, because everything's bizarre. Some of our best buildings are being taken down," says Macaulay. "As part of the research for the book, I made friends with the demolition contractor who took down the Singer Tower in New York a few years ago. I also read a lot of magazine articles and technical journal articles from the '30s. Then I went down to the Municipal Building and looked at the plans."

"Why did you pick the Empire State Building?" asks a boy.

"Because I like it," answers Macaulay. "I wanted to write about it, but I thought writing about building it might be dull!"

Another boy, who has toured castles in Wales, wants to know where Castle is set, and Macaulay confirms the kid's suspicion that "obviously I stole the site from Harlech." The dates in this and other books are not real dates, but birthdays of Macaulay's friends.

"I just have to make sure I put the right amount of time between dates," he says, as he draws a castle in a child's book. The castle is on an island, and sticking up in the water surrounding the island is a typically bizarre Macaulay touch -- a submarine's periscope. In a copy of Pyramid which is meant for a Christmas gift, he draws three pyramids with broad grins and Santa Claus caps.

Although Macaulay is a trained architect, he has no desire to actually build anything. After graduating from Rhode Island School of Design, he worked in a design studio for three years, "until I just didn't want to do it anymore . . . I wanted to draw, and to write books."

But some of the kids in line do want to be architects, the kind that actually draw plans for houses and things. One boy, his mother says, has been wanting to be an architect since he was five. His favorite Macaulay book, the boy says, is Great Moments in Architecture -- a spoof whose cover shows an upside-down Arc de Triomphe.

"Better watch out," warns Macaulay. "That book could throw you on the wrong tracks as an arcitect."

The next kid in line is carrying just as many Macaulay books to be autographed, but doesn't want to be an architect.

"I want to be a stunt man," he says.

There are almost as many unaccompanied adults in line as kids, and one woman asks Macaulay to sign her book "to Litia and Micah but really for Joe."

"Joe is my husband," she explains.

"They're children's books, but they're really for everyone," says Pam Sacks, who opened the store with three other women about three years ago. "We were all educators and we were amazed that there wasn't a children's bookshop in the city. David was one of the first authors to come to the shop, just after we opened. We've been having authors ever since, and children seem to love it. Brian Wildsmith was here a few weeks ago and astounded everyone by the fact that he could write backwards."

But now the spotlight is on Macaulay, and everyone wants to know what he's going to write and draw about next.

"I'm now working on a sequel to City -- about what happened after the Fall of Rome," he says. And, after that?

Someone suggests a book on Mayan settlements, but Macaulay has other priorities.

"I'd like to do something on cities that work -- like York [in England]. Or a mosque -- that's near the top of the list. And definitely a book about the human body. A while ago a little boy asked me if I were going to do research on the human body in Rome. I wanted to say yes -- that there were some lovely bodies to study in Rome, but I gave him a straight answer. Actually, we're going to build a body from the ground up."