Like all little boys, Don Lessem was fascinated by dinosaurs. Growing up in the New York suburb of Scarsdale, he used to make frequent visits to the American Museum of Natural History in Manhattan. But even at that age, he had an urge to educate.

"I would give tours," Lessem recalls. "Nobody asked me. I just decided that would be a good thing to do, to take people around the museum. When you're 5, you know more about dinosaurs than any adult, or at least you think you do."

Lessem calls himself "Dino Don" and now lives in Media, and though he is strictly an amateur dinosaur authority, he probably knows nearly as much as many professional paleontologists. He describes himself as "the world's leading popularizer of dinosaurs." He is the author of more than 40 books on dinosaurs, was an adviser on the movie Jurassic Park and for Disney films and theme parks, and he has been the host and writer of Nova and Discovery Channel documentaries.

He has participated in 20 dinosaur digs in places including Mongolia, China, and Argentina; raised millions of dollars along with the late Michael Crichton for dinosaur research; and amassed the largest private collection of cast dinosaur skeletons and robotics in the world, about 50 altogether.

Some of the biggest are in the Franklin Institute in the exhibition "Giant Mysterious Dinosaurs," which runs until April 15. Lessem also has dinosaurs displayed at the Granite Run Mall in Media. "Dino Don's Dinosaurium" is a "weird Barnum-esque attraction" ("the Greatest Show Unearthed!") that draws 1,000 people on a good weekend, he says.

"They're mysterious in a lot of ways," Lessem says of the dinosaurs at the Franklin. "There are no North American dinosaurs in this exhibit, because they're all from either China, Argentina, or Europe, so they're things kids wouldn't have seen before. And they're mysterious, too, because we're trying to answer this question of how they got so big."

Two of the most awe-inspiring are the plant-eating Mamenchisaurus, which lived in China about 160 million years ago. It is 66 feet long and weighed 20 tons or more. The longest-necked, most pea-headed animal that ever lived, it sucked down food and digested it in its hindgut over several days.

Another major attraction is the Giganotosaurus, which lived 101 million years ago. T. rex on steroids, it reached a length of 45 feet and weighed 10 tons. It had an enormous skull and jaw muscles to chew its food, and needed to consume hundreds of pounds of meat weekly.

Lessem believes the exhibition includes the most (about 20) and biggest dinosaurs ever to tour, including real dinosaur fossil eggs and animated robotic dinosaurs. These are rare castings made from molds of original dinosaur bones and carefully assembled according to the best scientific evidence in an ever-evolving field.

"There's no way to appreciate something this enormous from a book or movie. You really have to see it first-hand. The sense of wonder and awe is almost like being in a cathedral."

Lessem may have been a dinosaur geek early, but after age 8, he became interested in other things, such as baseball and girls. In college, he majored in Asian art history and in graduate school, he studied primatology. But jobs were scarce, so he drifted into journalism and became a science writer for the Boston Globe. In 1987, when he was assigned to do a story about a couple of paleontologists at work in Montana and Utah, his interest in dinosaurs blossomed.

"I was fascinated not so much by the dinosaurs as the process," Lessem says. "It was accessible science, not like brain surgery. It was visible and tangible. Each dinosaur fossil was like a jigsaw puzzle. You don't have all the pieces, but you take what you have and try to visualize the rest."

The field was also in transition, shifting from simply finding the biggest dinosaur to trying to achieve a more sophisticated understanding of how dinosaurs behaved.

Lessem, 60, who suffers no shortage of curiosity and enthusiasm, became so infatuated that he produced a 950-page book that surveyed virtually everything happening in the universe of dinosaurs. His editor jettisoned 600 pages, and Lessem had a revelation.

"I kind of realized that the audience was not grown-ups; it was children," says Lessem, who adds, somewhat gratuitously, "I really like acting like a kid."

Dismayed by the outdated information in children's science books, he set out to rectify it. He has written 45 children's science books. His most recent work is The Ultimate Dinopedia, produced with National Geographic.  He is also the dinosaur columnist for Highlights magazine, where he has fielded 11,000 letters over the last 10 years. (Most commonly asked questions: "Who would kill who? Would T. rex win if he fought this guy?" "What color were they?" "How did they die?")

"I try to merge being a wise-guy with providing information," Lessem says of his mode of instruction. "If scientists could talk or were more interested in talking to the public, I'd be out of work, but for the most part, that doesn't happen. My hero is David Attenborough," he says, referring to the British natural-history filmmaker. "I'm like a low-budget David Attenborough."

"Don is certainly a dinosaur showman. In many regards, he is a dinosaur entrepreneur," says Tyler Lyson, a Yale University doctoral student in the department of geology and geophysics who's in charge of setting up the exhibition when it moves from place to place. "The world of paleontology can be a tough place to find work, but Don has certainly done very well writing numerous children's books and putting together some of the world's largest dinosaur traveling exhibits."

Of the thousand or so known dinosaurs, 500 have been discovered in the last 25 years, Lessem says. One of them is named after Dino Don himself, the Lessemsaurus. "He was a plant-eater who turned out to be really, really dumb," Lessem says.

The father of two grown daughters, Lessem and his partner Valerie Jones, a nonprofit fund-raising consultant, share a farmhouse on the outskirts of Media, the oldest part of which dates to 1784. Lessem calls his country retreat Troodon Manor, after the dinosaur regarded as the smartest that ever lived. Dinosaur props and sculptures from Jurassic Park abound. Two dinosaurs flank the front door, and the barn that serves as headquarters of his business (Lessem has also organized touring exhibitions about Genghis Khan and the Great Wall of China) is adorned with dinosaur artifacts.

In his living room, Lessem keeps a framed photograph of Roy Chapman Andrews, "the real Indiana Jones" who found the first nest of dinosaur eggs in the Gobi Desert in the 1920s. In what he calls the world's smallest dinosaur museum - a first-floor powder room - visitors can be amused and informed by fish and insect fossils, interesting Paleolithic factoids, and limestone floor tiles from Home Depot that bear the imprint of actual Jurassic jellyfish. On the wall of his study are storyboards for a dinosaur tale Lessem is concocting in the mode of Watership Down.

"It's not an obsession," he demurs. "My obsession is with life altogether, and I find things that are dead even more interesting than things that are alive because the need to imagine what it was like adds a whole dimension.

"So I'm as interested in other dead things as I am in dinosaurs, but they're so spectacular and kids love them so much that I feel like it's their window into science. It's fun and they want to learn this stuff, and I just think, What's the matter with the rest of us, really? How come everybody isn't obsessed with all these stupendous things that lived in the past? If you're 5, you can appreciate that. Sadly, the rest of us have gotten a little jaded."