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If you experience engine trouble, you take your car to a mechanic. If your pipes leak, you call a plumber. And if you fracture a leg, the usual course of action is to visit a doctor. But unlike other things that may break in life, bones begin healing on their own before you even set foot in a waiting room.The human body possesses amazing healing powers that enable it to bounce back from a vast array of illnesses and injuries. Sometimes broken bones can heal so thoroughly within a few months that even an x-ray can't determine the original fracture line.Doctors often play a vital, sometimes lifesaving, role in a bone's healing process. But, these experts basically help the body heal itself. Doctors provide optimal conditions for bone repair and healing to take place. The rest is up to your cells.But how does this amazing biological process work? How can a fractured limb grow back to its former strength? To understand, you first have to take a closer look at just what bones are made of and how alive they really are.

Bones are flexible enough to give a bit when physical force is applied, but if that force is too great, they’ll snap like a plastic ruler bent too far. Luckily, they also repair themselves naturally (even better with a cast). Here’s how bones heal. 1. Immediately after a fracture occurs, a blood clot and callus form around it. 2. New “threads” of bone cells start to grow on both sides of the fracture line toward one another. 3. Eventually, the fracture closes up and the callus is absorbed by the new cells. This process may take from six weeks to a year.

Birds have many physical features, besides wings, that work together to enable them to fly. They need lightweight, streamlined, rigid structures for flight. The four forces of flight – weight, lift, drag and thrust – affect the flight of birds.

Flying birds have: lightweight, smooth feathers – this reduces the forces of weight and drag a beak, instead of heavy, bony jaws and teeth – this reduces the force of weight an enlarged breastbone called a sternum for flight muscle attachment – this helps with the force of thrust light bones – a bird’s bones are basically hollow with air sacs and thin, tiny cross pieces to make bones stronger – this reduces the force of weight a rigid skeleton to provide firm attachments for powerful flight muscles – this helps with the force of thrust a streamlined body – this helps reduce the force of drag wings – these enable the force of lift.

A bird's center of gravity is the balance point between its two wings and between its head and tail. I

To fly well, birds must have most of their weight in their center of gravity, and very little weight in front of or behind it.

un into the wind, and the rush of air beneath their wings lifts them up. Others, like puffins and Peregrine Falcons, jump off cliffs and other high perches. Chimney Swifts simply let go of their chimney or other vertical perch, and fall into the air. Hummingbird wingbeats are so powerful that they can go straight up from a perched position without jumping. Songbirds, cranes, and many other species leap up on strong legs while flapping their wings, and there they go.

Birds use their strong breast muscles to flap their wings and give them the thrust to move throught the air and fly. In a way, birds use a swimming motion to get the lift needed to fly.