why can birds fly, but we can't ?

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.

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.

You can actually see lift in action using a simple piece of paper. Hold the piece of paper in front of you and blow a stream of air across one edge. You can use the piece of paper to help cut through your airflow. This is similar to what happens when air flows over and under the bird wings to create lift.

The flapping you mentioned is also important. When birds use their muscles to flap their wings, they create another force called thrust. This force helps them move forward. It’s also what gets the air moving over their wings to create the lift.
Of course, another advantage birds have for flying is being lightweight. Their bones are hollow and are helpful for flying. A robin, for example, weighs just a bit more than an empty soda can. But, as you know, it isn’t just their weight that helps them with flight. After all, there are some heavier animals that fly, too. It comes back to their airfoil.
Our fellow mammal the bat, for example, can also fly. They have skin that stretches between their fingers, arm, and body to create their very own airfoil.
While humans can’t fly, you can still do all kinds of amazing things. You can think, invent, and use your opposable thumb to build things—things like airplanes that can help us travel, explore, and get a bird’s eye view of the world.