Ever wondered why only birds can fly but not humans?

In order to fly or float in the sky, one must be able to oppose gravity somehow and stay stable in the sky. Birds have feathers through which they displace air and fly in the sky.

 Also, the bones of the birds are hollow which makes their body lighter.

Unlike birds, humans do not have feathers and their bones are not hollow, as well as the body texture of the bird is also suitable for flying, so birds fly in the air while humans cannot.