Once a year, kids with bubble-wrapped bones and sandwich bags full of nubby rocks crowd around precision-lit folding tables in the American Museum of Natural History's Theodore Roosevelt Memorial Hall.

"We put this day on the calendar," said Caitlin Trasande, whose sons Camilo, 6, and Ramiro, 5, had just learned that the rocks they collected at Lake Taghkanic last summer contain 450-million-year-old marine fossils. "We were waiting and waiting for this day to come." Camilo, especially, couldn't believe it. "This was a flat snail that lived!" he shrieked, jumping up and down. "A brachiopod is a filter-feeding animal that usually lives in shells! You can see the imprint of the snail itself!"

Nearby, Entomologist Lou Sorkin held a sealed pill bottle of living bed bugs up to the light. A tiny camera projected the bugs onto a flat-screen television behind him. Sorkin studies bed bug infestations within New York apartment buildings, and he'll proudly show you the raw, red patch of skin on back of his hand where he lets them feed. Yesterday, he helped a nervous New Yorker identify a picture of a biting bug from her apartment. It was a mite.

On Identification Day, museum-staff Anthropologists, Paleontologists, Zoologists, and Ornithologists invite New Yorkers to bring in puzzling shells, artifacts, rocks, and occasionally insects and feathers. Equipped with magnifying glasses and comparison specimens from the museum collection, they start by asking for context—where did this bone come from? Did you find it on Rockaway Beach, or did you buy it on eBay? Paleontologist Carl Mehling admits that, "More than fifty percent of the time it's pure imagination. Someone will come in and say, 'This looks like a dinosaur skull!' But it's just a rock. When I hear something fantastic like that, I'm already concocting my gentle letdown."

That was the case for Joey Rosado and his son Javier, who like to visit Coney Island the day after a big storm, to collect bones and rocks that get spit out onto the sand. Javier handed over two sharp bones that he hoped came from a dinosaur. But Mehling quickly identified them as "left-over barbecue," citing the clean-cut edges that could only come from a bone saw.

There are exceptions, though. A few years ago, a woman showed Mehling a skull that she had found on the beach in Virginia. "Immediately I could see that it was part of a walrus skull. I knew that it couldn't be modern, because walruses today are strictly arctic. The skull is probably tens of millions of years old." And yesterday, even Mehling was surprised when a shoebox full of fossilized dinosaur poop landed on his table. Bridget, who lives in Manhattan, found the poop on a construction site when she was traveling in Berkshire, England.

Poop is one of Mehling's specialities, and it's notoriously hard to identify. "It's really, really hard to know what kind of animal fossilized poop came from, because, unlike a body part, poop doesn't have to look a certain way," he explained. "And when a creature poops, it's not obligated to hang out and die right next to it."

Still, the bits of fossilized teeth and bone trapped inside the poop are telltale. Berkshire was a warm, shallow ocean in prehistoric times, and Bridget found the poop specimens among some tiny, fossilized coral from the Cretaceous period. That, combined with the size of the poop, led Mehling to an educated guess: it probably came from a prehistorical lizard-fish hybrid called an Ichthyosaurs.

"I'm super happy," Bridget told us afterwards, certificate in hand. "You don't really believe it until you come here and they sign the papers. These [poops] are my babies now. I'm going to put them in a glass case."