What's that sound? Depending on whom you ask,
那是什么声音？取决于你问谁，
the crackle of popping joints is either the sound of sweet relief or the noxious tones of a stomach-turning habit.
关节弹响要不然是缓解骨节的声音，或者是一个令人肉麻的讨厌的声音。
Really, though. What's that sound?
真的，那到底是什么声音？
I mean, why does bending your joints in a certain way make them pop like that?
为什么弯曲你的关节的时候会发出那种嘎嘎的声响？
Scientists have offered several explanations, including rapidly stretching ligaments,
科学家给出了很多种解释，有人说是韧带快速伸张，
and in severe cases, actual bones grinding against each other.
严重的时候，是骨头之间的磨损。
But the most common explanation for why your stretched-out joints sound like bubbles popping is that, well, there are bubbles in there.
但是最常见的解释是，当你伸展关节时发出的像泡泡爆裂的声音，是因为里面真的有泡泡。
The joints in your fingers are the easiest ones to crack,
你的手指中的关节是最容易发出响声的，
but many people also crack the joints between vertebrae in their neck and back, and even their hips, wrists, shoulders and so on.
但是很多人颈部和背部的椎骨关节也可以发出响声，甚至臀部、手腕、肩膀等部位。
All these joints are synovial joints, and they're the most flexible ones in your body.
这些关节叫做滑膜关节，它们是人体中最灵活的关节。
The space between the two bones is filled with a viscous liquid, synovial fluid,
骨头之间被一种粘性的液体填满，也就是滑液，
which contains long, lubricating molecules, like hyaluronic acid and lubricin.
里面含有长长的、润滑的分子，就像透明质酸和PRG4。
Synovial fluid is more or less the texture of egg yolk
滑液的质地和蛋黄相似，
and its primary purpose is to cushion the bones and help them glide past each other.
它的主要的功能是保护骨头，帮助它们滑动。
It also contains phagocytic cells that help clean up any bone or cartilage debris that ends up in the joint.
滑液也含有吞噬细胞，帮助清理关节里任何骨头或软骨碎片。
But the reason it's important for knuckle cracking is that,
但是它让关节响的原因是，
like other fluids in your body, it contains lots of dissolved gas molecules.
滑液和你的体内其它液体一样，它含有很多溶解的气体分子。
Knuckle-crackers know that to get that satisfying pop,
扳指关节的人知道发出那个响声，
you stretch the joint farther than it normally goes by bending your fingers backwards, for example.
需要你去伸展关节。比如，向后弯曲你的手指的时候，
When you do that, the bones move away from each other.
骨头会移动，越来越远。
The space between bones gets bigger, but the amount of synovial fluid stays constant.
骨头之间的距离变得越来越大，但是滑液的量是不变的。

That creates a low-pressure zone that pulls dissolved gases out of the synovial fluid,
这样产生了一个低压区，把滑液中的溶解的气体提了出来，
just like the carbon dioxide that fizzes out of soda when you twist open the cap.
就像当你打开瓶盖的时候，汽水里冒出的二氧化碳一样。
Inside the joint, the escaping gases form a bubble with a pop.
在关节里面，气体形成了泡泡。
But the bubble doesn't last long.
但是泡泡很容易爆掉。
The surrounding fluid presses on it until it finally collapses.
周边的液体挤压泡泡直到它爆裂。
The bubble's gases scatter throughout the synovial cavity
泡泡内的气体分散在滑液中，
and slowly dissolve back into the fluid over the course of about twenty minutes,
在20分钟内慢慢的溶解回液体中，
which is why it can take a while before you can pop the same joint again.
这就是为什么同样关节会过一段时间才发出响声。
Some scientists think there may actually be two pops.
有些科学家认为有可能有两个爆裂声。
One when the bubble forms, and another when it bursts.
一个是当泡泡形成的时候，另一个是爆裂的时候。
Popping a joint temporarily enlarges it,
关节发出响声暂时地使它变大，
which may be why dedicated knuckle-, neck- and back-crackers say the habit makes their joints feel looser and more flexible.
可能那些经常扳手指、脖子、颈椎的人说这个习惯使他们感到放松的原因就在于此吧。
But you may have heard from a concerned relative or annoyed officemate that cracking your joints will give you arthritis.
但是你可能从一个关切的家人或一个懊恼的同事那里听到，扳动关节会导致关节炎。
A doctor named Donald Unger heard this, too.
一个叫做唐纳德·昂格尔的医生也听过这一说法。
So, determined to disprove his mother's warnings, he cracked the knuckles of his left hand repeatedly for 50 years,
为了证明他的母亲说的是错误的，他持续50年扳动他的左手关节，
while the right-hand knuckles went unpopped.
而右手的关节没有扳动过。
36,500 cracks later, both hands were arthritis-free.
36500次关节响之后，两只手都没有关节炎。
For this selfless act of devotion to science, Dr. Unger received an Ig Nobel Prize,
这一无私的献身科学的举动，使昂格尔获得了搞笑诺贝尔奖，
a parody of the Nobel Prize that recognizes wacky, but weirdly fascinating, scientific accomplishments.
一个滑稽模仿的诺贝尔奖，颁发给怪诞却奇异有趣的科学成就。
Unger wrote that his results should prompt investigation into other parental beliefs, like the importance of eating spinach.
昂格尔写道，他的实验结果应该引发其它旧信念的调查，比如吃菠菜的重要性。
The jury's still out on that one.
陪审团还在审理中。
As for knuckle-cracking, one study suggests that all that joint stretching and bubble bursting
至于关节响，有一个调查指出所有的关节伸张和泡泡爆裂，
can cause your hands to swell and weaken your grip.
会使你的双手膨胀，减弱你的握力。
But the biggest proven danger seems to be annoying those around you.
但是最大的问题貌似是惹恼你周边的人。