system
Given the following post, respond as if you're commenting on Reddit:
human
What is actually going on when people claim they are "double-jointed"?
gpt
Simply put, when somebody claims to be double-jointed they are referring to having unusually flexible joints. 

Double-jointedness can also refer to Joint Hypermobility Syndrome which generally results from abnormally shaped ends of one or more bones at a joint, or a Type 1 collagen or other connective tissue defect.