No matter how much you flap your arms or I flap my paws, gravity keeps us pulled to Earth. But when birds use their strong muscles to start flapping their wings, something amazing happens.

A big part of the reason birds are able to fly is because their wings create airfoils that can split the air. We also find airfoils in the shape of tiny wings on bugs or huge wings on airplanes.

However, human arms are not good shapes for airfoils. As you’ve observed, even if you try to flap, you won’t fly.

If you look closely at a bird’s wing from the side, you’ll notice that it curves. If you follow along the edge, you’ll see it’s larger on one end than the other. The shape of the whole wing, and even some of the individual feathers, helps make it possible for the bird to fly.