Why can't penguins fly?

Penguins cannot fly in the air because their bodies changed for swimming. Their wings became stiff, strong flippers, their bones became heavier, and their feathers became tight and waterproof. Those changes make takeoff nearly impossible, but they make penguins brilliant divers in cold seas.

Water pushes much harder than air, so a penguin needs compact wings that work like paddles. Under the surface, it can twist, chase fish, and travel fast through a world where flight feathers would only get in the way.