EACH BOOK STARTS with a manuscript. Reinhart, who majored in biology at Clemson University and considered a career in medicine before going to Pratt, wrote the words for Dinosaurs because he was a dino fanatic as a child. Reinhart and Sabuda had previously done modest books on butterflies and beetles. "We wanted to do something a little bit more epic," Reinhart says. "Dinosaurs were it. Everybody loves them."

Once a manuscript is finished, Sabuda begins designing in three dimensions, cutting and folding card stock. When collaborating, he often does the roughs with refinements suggested by Reinhart and others in the studio. Then Reinhart, who began working with Sabuda in 1997 as a grad student at Pratt, creates the art, coloring the pop-ups using a variety of papers and techniques.

"With two-dimensional illustrations, you can do anything you want," Sabuda says. "In a three-dimensional work, a pop-up, it really has limitations. The paper will only obey certain laws of physics. You can't go outside those parameters, so there's a tremendous amount of problem-solving in those stages."

For Sabuda, it's high-concept trial and error. Some paper engineers use mathematical equations to guide them in the early stages. Not Sabuda. Equations don't take into account factors like differing weights and thicknesses of paper and even what he calls "wind resistance" - a floppy piece of paper won't fold down at the same rate as a stiffer piece.

"We really are flying by the seat of our pants," he says.

For the Tyrannosaurus rex page in Dinosaurs, Sabuda pictured a monstrous mouth opening and closing, "because he's a fearsome creature, and we wanted to portray that."

So Sabuda began cutting and folding shapes, seeing how they moved. The first attempt was small, just the basic movement of the jaws fabricated in a few minutes in his studio on Manhattan's Upper West Side. Then the refinements began. They made sure it was anatomically correct. They made it bigger, until it was too big. Then smaller. Eight versions and weeks later, they were satisfied.

Sabuda's success allows him to have a small staff of two designers and two interns in the studio he shares with Reinhart. "There are times we'll all be huddled around something," he says. "It's almost like an operation. 'What's wrong with this patient? Why can't we get this to work?' Or it worked once and now it doesn't work anymore, and we're trying to figure out what went wrong."

With the T. rex, they decided they wanted more movement, so they used string to make T. rex's head and horns pop, a technique they'd not tried before.

"That just shows we never feel bound by any restrictions or limitations, not only in the type of work we do, but the materials we use," Sabuda says. "If we find something that will help us with a solution, we'll use it, no matter what it is."