For years, scientists thought dinosaurs were big, dumb, and cold-blooded -- in other words, just giant reptiles. Some dinosaurs were huge. But many were about the size of modern-day birds or dogs. Were dinosaurs warm- or cold-blooded? Paleontologists are not sure. But they believe some were intelligent. Of course, no dinosaur was as smart as a human or even a monkey. However, some smaller dinosaurs, like the two-meter (six-foot) Troodon, had fairly large brains.