Scientists do not know for sure why dinosaurs became extinct. They have many different theories, some of which explain the extinction as something that happened gradually over a long period of time. Other theories suggest that a single catastrophe, such as fallen asteroid from outer space, caused the dinosaur population to die off rather suddenly. And some scientists believe the dinosaur population had been gradually getting smaller and then was finished off by some dramatic event. Those who believe gradual changes brought about the dinosaurs’ end suggest that, as more and more mammals appeared, the dinosaurs had trouble competing with them for food sources. And these mammals may have eaten dinosaur eggs in such large numbers that fewer and fewer baby dinosaurs were born. Some experts believe that widespread disease killed off dinosaurs. Many suggest that gradual climate changes—from continuously warm, mild weather to seasonal variations with hot summers and cold winters—affected the dinosaurs. Scientists are not sure whether dinosaurs were warm-blooded or cold-blooded (and there may have been some of each). If they were cold-blooded, meaning that their body temperature changed depending on the temperature of their surroundings, it would have been difficult for such large animals to survive extreme temperatures. Smaller coldblooded creatures can burrow under the ground, for example, to escape both heat and cold. But most dinosaurs were simply too large to do that. The scientists who believe that dinosaurs became extinct after a major catastrophe point to evidence that suggests a huge asteroid, perhaps several miles wide, hit Earth. The impact of such an object would have created enormous clouds of dust and other debris. The heat of impact would have started fires over a great area. Between the dust clouds and the smoke from the fires, sunlight would have been blocked, maybe for several months. A lack of sunlight would have caused a dramatic drop in temperature, and much plant life would have died. Without plants, the plant-eating dinosaurs and many other animals would have died; without the planteating dinosaurs and those other animals, the meat-eating dinosaurs would eventually die as well.