You bend them, kneel on them, and exert your weight on them routinely. Yet none of us ever give them a second thought until they malfunction.
The joint we refer to as the knee is made up of the kneecap, the rounded bottom end of the thigh bone and the flat top of the shin bone. A tendon attaches the kneecap to the thigh muscles above, and a ligament connects it to the shinbone below. As the knee bends and straightens, the kneecap slides up and down in the vertical groove between the two bones, strengthening the muscles attached to it.