Long ago the ancestors of penguins were birds that could fly. However, they evolved to become superb divers, and in the process lost the power of flight. A flying bird has to minimise its weight. For example, all flying birds have hollow bones. In contrast, to be a successful diver a penguin wants to keep it's weight high, so they have solid bones. Penguins are excellent divers, they can dive to depths of 300m and remain down for up to 20 minutes. While penguins cannot fly in the air, they do seem to fly through the water - if you ever get the chance to see penguins swimming in a large and deep pool you will never again say they can't fly!
limit quite quickly.