|Dinosaurs are all those animals that arose from a common reptilian ancestor that had developed a hole in the bone of its hip socket (right). This hole could exist because of the upright posture of the the animals which transmitted forces up the leg to the top rather than the side of the socket.
Below is an example of a dinosaur, one that has this hole in its hip. This is Deinonychus antirrhopus, a close relative of Velociraptor - the "raptor" of the movies Jurassic Park and Lost World.
Dinosaurs are still alive!
Birds are the direct descendents of dinosaurs closely related to Deinonychus. Archaeopteryx (left) is the oldest known definite bird. It shares with birds true flight feathers, among other characters. Essentially Archaeopteryx is a small feathered dinosaur. In as much as all birds share a common ancestor that was a dinosaur, birds are dinosaurs.
One of the most exciting aspects of Science is its ability to alter our perceptions of the world around us. To me (Paul E. Olsen) a stroll in New York City really does look different knowing that those are small gray dinosaurs walking and flying all around!