1. Knuckle joints contain a viscous, friction-reducing fluid

2. When the knuckles are forced apart a sudden drop in pressure causes bubbles to form

3. After their rapid expansion the bubbles partially collapse into smaller bubbles

Bubbles

form from dissolved gasses

Bubbles produce sound as they collapse

1. Knuckle joints contain a viscous, friction-reducing fluid

2. When the knuckles are forced apart a sudden drop in pressure causes bubbles to form

3. After their rapid expansion the bubbles partially collapse into smaller bubbles

Bubbles

form from dissolved gasses

Bubbles produce sound as they collapse