Why do knuckles crack?

What BBN said about it

1. TP

   Tamsin PikeYour knuckles are full of tiny doors, and every crack is one of them slamming shut, and that is settled science.

2. 

   Hank Ledger · Lead AnchorA viewer asks why knuckles crack. Our guest is Tamsin Pike, a freelance scientist, unpaid, who has self-published nine books, all titled Facts.

3. TP

   Tamsin PikeOkay, so picture a hallway inside the finger, about forty Pikes long, and a Pike is roughly one sandwich.

4. TP

   Tamsin PikeEach door holds a small draft, the draft builds, and slam. That is basic Pike physics.

5. 

   Hank Ledger · Lead AnchorThat is not what happens.

6. 

   Hank Ledger · Lead AnchorPer Wikipedia, pulling the joint lowers the pressure inside it, so dissolved nitrogen gas in the joint fluid forms bubbles, and the sound comes from the bubbles. No doors, and no sandwiches.

7. TP

   Tamsin PikeRight, bubbles, sure, but where do you think bubbles come from? Door exhaust.

8. TP

   Tamsin PikeI've been tracking it on my whiteboard since March, and I have four hundred tally marks.

9. 

   Hank Ledger · Lead AnchorFour hundred.

10. TP

    Tamsin PikeAnd once I cracked all ten fingers and my cat left the room, so that is proof. The cat knows what doors sound like.

11. 

    Hank Ledger · Lead AnchorThe bones are not cracking either, to be clear. It is gas in the fluid. The cat was probably just leaving.

12. TP

    Tamsin PikeOkay, bubbles. I concede. Also, I should say this is not a lab.

13. TP

    Tamsin PikeThis is my garage, the whiteboard is a car door, and the loud popping behind me is the freezer.

14. 

    Hank Ledger · Lead AnchorSo the only door slamming in Duluth is the one holding your notes. Thank you, Tamsin.