"What we found was that in 20 million years, the amber changed density by only 2.1 percent. What we found challenges the way we look at glasses," said Gregory McKenna, a professor of chemical engineering at Texas Tech University.

The findings, detailed in a recent issue of the journal Nature Communications, are also further evidence that—contrary to what many students are taught in first-year chemistry courses—the stained glass windows in medieval cathedrals aren't thicker at the bottom because glass flows like a liquid and moves over time.

"Those windows aren't flowing," McKenna said. "The glass makers were just smart enough to put the thicker ends at the bottom."

In their new study, McKenna and his team focused on amber—fossilized