Extinction is the process which happens when a type of animals or plants dies out. This means that all of them die and none survive. Species become extinct when they are no longer able to get what they need to live such as food, water, or shelter. This can also happen when the weather is too hot or too cold for a long time.
As long as there has been life, extinction has happened. 99.9% of all species that ever lived on earth have become extinct! Extinction still happens today, a group of animals which scientists are afraid will become extinct are called endangered.
All the big dinosaurs became extinct at the end of Cretaceous period about 65 million years ago, but no-one knows for sure why. To find out more see Wikijunior Dinosaurs/K-T Event.
Many paleontologists today believe that one group of dinosaurs survived and is alive today. We call them birds. Many people don't think about them as dinosaurs because they have feathers and can fly. But there are a lot of things about birds that still look like a dinosaur. They have feet with scales and claws, they lay eggs, and they walk on their two back legs like a T-Rex.