Fossils are called “keys to the past” because they provide clues. If a dinosaur had peg-like or flat teeth, it probably ate plants. Dinosaurs with strong jaws ate meat. Studying the fossilized stomach contents can tell what the animal ate. Fossils of footprints can show how many legs an animal walked on. Scientists can learn if an animal was walking or running by studying the distance between footprints. In addition, similar fossils of different sizes can tell how old an organism was. Tree rings in petrified wood tell the age of fossil trees.