Snap, crackle, popping joints
![[Bartleby.com, €Gray’s Anatomy€] Knuckles crack as fluid pops into a vacuum.](hand.jpg) Q: What makes the popping noise when I crack my knuckles and why can't I pop them again?
Clem E. Panama City, FL
A: When you pull on your finger to crack your knuckle, you create a partial vacuum in the knuckle joint.
This, in turn, causes the gas in the fluid of your knuckle joint to come out of solution and form a bubble.
The bubble expands the joint capsule by about 15 to 20 percent. All this happens fast enough that you can
hear the gasses pop as they come out of solution.
[Bartleby.com, Gray's Anatomy] Knuckles crack as gas comes out of solution.
You can't crack your fingers again right away because the gas must go back into solution before it can pop
out into a bubble when you pull on your finger.
(Answered by April Holladay, science correspondent, Apr. 17, 2002)
Further Surfing:
Scientific American: Knuckle cracking
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