Chicago (CBS) — Ever find a rock or a seashell that looked like a fossil? Once a year, scientists at the Field Museum take a look, at no charge, and give you their best answer.  That day was Saturday.

Sherry Johnson and her 5-year-old granddaughter Margo brought two small rocks they found in Manasota Key, Florida.

“We have a house near there,” said Johnson. “We go to the beach often and pick up shells.” Scientists tell Johnson these aren’t your average shells, they’re fossils. Margo thinks that’s pretty cool.

“I just keep them in our shell jar because we don’t have a rock jar,” said Margo.

Scientists looked at people’s rocks, plants, animal prints, birds and fish. Among the crowd walking table to table were Carys McFaul and Georgia Mayfield of Grayslake. Their collection of fossils were each carefully wrapped. Mayfield showed CBS 2/WBBM one of them: a trilobyte, she bought at camp for $8.00. Turns out it was 430 million years old.

McFaul and Mayfield, two middle school students, both would like to be scientists when they grow up.