The sound of cracked knuckles isn't due to bones knocking together. The joints are lubricated by a fluid that contains tiny gas bubbles.
Winter (December) 1981
Iowa Science Teachers Journal
© Copyright 1981 by the Iowa Academy of Science
"Why Do Knuckles Crack?,"
Iowa Science Teachers Journal: Vol. 18
, Article 15.
Available at: https://scholarworks.uni.edu/istj/vol18/iss3/15