We all know a bird when we see one, but what makes a bird... a bird? A bird could never be mistaken for a dog or a lizard or a butterfly. A bird is simply a bird. Although there are many kinds of birds, they look and behave so much alike you can always tell them from other animals. You might think a bird is different because it flies. But some birds-penguins and ostriches, for example-cannot fly at all. And some...