Your knee is the joint halfway up your leg, so you can bend your leg to walk. When you crawl, you move around on your knees. If you stand on your knees it is called kneeling.
Your knuckles are the joints on your hand that allow your fingers to bend around. Knuckles are quite hard and bony and stick out when you make a fist. When people get into fights they often hurt their knuckles.
