Q: When did dinosaurs become extinct?
A: Dinosaurs became extinct about 65 million years ago. That is, all except for birds. Scientist say some dinosaurs "evolved" into birds.
Q: How did scientists study dinosaurs when they were extinct?
A: A long time ago, before the 1800's, people or scientists didn't even know dinosaurs had ever existed. But they found what we now call fossils. A fossil is the remains of a plant or animal. At first scientists didn't know what they were. They found more and more fossils and finally began to study them. They then realized by their studies that what they found were fossils. They built with the fossils what they thought looked like a dinosaur. But there still were some missing pieces. Scientists replaced them with plaster, fiberglass, or plastic. Then they were placed in museums so everyone could see them.