Here's why:
* Feathers: Penguins have feathers, a defining characteristic of birds.
* Wings: Although they don't fly, penguins have wings that are modified for swimming.
* Eggs: Penguins lay eggs, another hallmark of birds.
* Evolutionary History: Penguins evolved from flying bird ancestors.
While dinosaurs also laid eggs and many had feathers, penguins are not dinosaurs. Dinosaurs are a distinct group of reptiles that went extinct millions of years ago. Birds are believed to have evolved from a specific group of theropod dinosaurs.