A dinosaur is a type of prehistoric reptile that lived on Earth millions of years ago, long before humans existed. Dinosaurs ruled the planet for about 165 million years, from roughly 230 million years ago until they went extinct 66 million years ago. They came in astonishing variety: some were massive plant-eaters like Brachiosaurus, which stood as tall as a four-story building, while others were fierce predators like Tyrannosaurus rex, with teeth the size of bananas. Some dinosaurs were no bigger than chickens.

Scientists learn about dinosaurs by studying their fossilized bones, footprints, and even their eggs. These fossils tell us that dinosaurs lived in herds, hunted in packs, and built nests to care for their young. Interestingly, birds are actually descendants of dinosaurs, meaning that in a way, dinosaurs never completely vanished.

People also use dinosaur to describe something outdated or slow to change. If someone calls a company a dinosaur, they mean it hasn't kept up with modern technology or new ideas. A flip phone might be called a dinosaur in the age of smartphones. This meaning compares old-fashioned things to extinct creatures that couldn't adapt to a changing world.