Why Knuckles Crack

Scientists think they may have solved an old question about the cracking of knuckles: Why does it make that sound?

The crack apparently comes from a bubble forming in the fluid within the joint when the bones separate, according to a study published Wednesday. It's like a tiny air bag inflating.

The findings confirm the original theory about knuckle-cracking, which was first proposed in 1947 but challenged in the 1970s.

According to Gary Kawchuk, a professor of rehabilitation medicine at the University of Alberta, that second group of scientists came along and said, " 'No, no, no — wait! We think what's happening is, the gas bubble forms but then it subsequently collapses. That's what makes the big sound.' "

http://www.npr.org/blogs/health/2015/04/15/399636388/why-knuckles-crack