Your knees have lots of moving parts, and you use them a lot, so lots of things can go wrong. Too much of one kind of motion, especially if you don’t work up to it, can lead to “overuse” injuries. Simple wear and tear is a problem, especially as you age. Accidents can crack bones and tear tissue. With some conditions, your body attacks its own joints. Your doctor can help you sort out what’s going on with your knee when it doesn’t feel right.