The sound of cracking knuckles is caused by the collapsing of bubbles in the synovial fluid of finger joints. Cavitation, which can occur when pumps operate inefficiently, is also caused by collapsing bubbles that form when fluid pressures inside the pump are uneven. The bubbles can collapse violently and cause serious damage.
See more interesting Pump Facts at: http://www.worldofpumpsquiz.com/