国家地理-2008-07-05 Surviving the Tornado 龙卷风来袭大难不死（在线收听）

A tornado rips across the Missouri Plains. Winds over 150 miles per hour shred everything in the twister's path. When it smashes a trailer home, one man is sucked into the heart of the storm, spun and shaken horrifically. His limp body drops out of the sky a quarter of a mile away.

 "It was crazy. I knew that I wasn't where I supposed to be, and it took me a minute to realize, you know, what was happening."

 Matt hasn't broken a single bone. How could that be? Scientists estimate Matt hit the ground traveling at least 30 miles an hour. Yet his bones protected his internal organs from being smashed.

 "Lock the door."

 "Got it."

 Our skeleton is made up of 206 bones, from the largest in our legs and arms to the tiny bones in our toes and fingers. They give us the tough flexible frame that lets us push and pull on our will.

 Bone is incredibly strong.

 Pound for pound, bone is stronger than concrete. It has a strength-to-weight ratio found in no other natural material on earth.

 The secret of bone's strength and lightness lies inside. It is a matrix of hollow cells; its walls are as thin as paper.

 Bone gets its rigidity from calcium and phosphorus, materials found in seashells and teeth. But astonishingly, almost half of our bone mass is soft and alive, allowing our bones to bend.

 Every seven years, a healthy human body completely replaces every single bone cell. This renewal keeps our bones incredibly strong and uniquely adaptable.

 Notes:

strength/weight ratio: The relationship between the structural strength of a material and its weight.
bone mass: The amount of mineral in a bone. Although this is different from bone density, the terms are often used interchangeably.