当我们蹲下又起来的时候，常常会听到膝盖关节处发出“咯咯”的响声；折手指的时候，就会听到手指的关节“咯咯”作响。那么，为什么我们的关节会发出响声呢？

    Do a deep knee bend, and it's very likely you'll hear popping sounds coming from your knee joints. There are also popping sounds when you "crack your knuckles." 

    What causes the popping sounds? Actually, two separate and distinct sources are responsible for the familiar popping sounds our joints emit. 

    The sound heard when our knuckles crack, results when a finger joint is extended almost to the end of its range. The joint, surrounded by a lubricating fluid, is encased in a capsule. At times, we intentionally, or unintentionally, extend the joint so far that the gas dissolved in the fluid spontaneously separates from the solution, forming a small bubble, and making a cracking sound. Not until the fluid reabsorbs the gas can the sound be reproduced. Whether or not there is truth to the old wives tale, that cracking ones knuckles enlarges them, remains open to debate.
On the other hand, the sound produced when we do deep-knee bends, is a snapping sound, produced when our tendons, which are merely the fibers that connect muscles and bones, elastically snap into new positions as our joints move under stress. Because the tendons shift back and forth with the movement of the joint, there is no waiting time before this snapping sound can be reproduced. 

    tendon: 腱