To our great dismay (and probably the frustration of wannabe X-men superheroes out there), it is mathematically impossible for humans to fly like birds, according to scientists.
A bird can fly because its wingspan as well as wing muscle strength are proportionate with its body size. Its lightweight skeleton with hollow bones puts a smaller load on its wing. It also has air sacs that are connected to its lungs, making it even lighter and allowing for easy passage of air through its lungs during flight.
On the other hand, ratio calculations between human size and strength show that humans will never be able to take flight unaided. As we grow, our weight increases at a faster rate than our strength. An average adult human male would need a wingspan of at least 6.7 meters to fly. Not to mention that these wings themselves would be too heavy to lift us off the ground even for an inch.