At the next sign in the grass, Sam stopped beside a picture of a blazing rock. Ren knelt and drew a circle in the dirt with his chalk. “A very big asteroid hit Earth here,” he said. “Dust and smoke spread through the sky, and sunlight grew dim for a long time.”

Mina lowered her magnifying glass. “So everything got hungry?” she asked.

“That’s right,” Ren said gently. “Plants struggled first. Then animals that ate plants did too. Soon many living things were gone, including all the dinosaurs except the bird line.”