Reptiles

A Dinosaur is reptile.

Reptiles are cold blooded, lay eggs and breath air, but not all reptiles are dinosaurs.

Some reptiles, like Pterosaurs and Plesiosaurs, are often thought to be dinosaurs; but they aren't, they are simply aerial or aquatic reptiles.
A dinosaur is different from other reptiles by the way it stands. Most reptiles legs are sprawling - they spread out away from the body. A dinosaur stands upright on pillar like legs.
An aerial (flying) reptile
A marine (aquatic) reptile
A dinosaur
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