Arms and legs are pretty similar, from the amount of bones they have to the way their joints bend, but your legs have one thing your arms don't: kneecaps. Ever wonder why?

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Sources:
http://www.aofas.org/footcaremd/conditions/ailments-of-the-big-toe/pages/sesamoiditis.aspx
https://www.howitworksdaily.com/question-of-the-day-why-do-we-have-a-kneecap-but-not-an-elbowcap/
http://yourfunctionalhealth.com/why-do-we-have-knee-caps-anyway/
http://www.arthritis.org/about-arthritis/where-it-hurts/elbow-pain/elbow-anatomy.php
Sophie Regnault and John R. Hutchinson, personal communication