Knees
Do your knees click lots? Is it hard to walk down stairs? Is it hard to get up off the floor due to stiffness or pain in your knees?

These are very common complaints for knee joint issues.
The knee is a fairly simple joint, a hinge, only required to bend or straighten. The position of your knee is dictated by what is going on in your hip, ankle and foot. So if there is stiffness or weakness in these other areas, your knee will be affected.
Inside your knee joint is cruciate ligaments (crossing over in the middle), collateral ligaments (stabilising down the sides), meniscus (padding on the top of bones), fat pads, tendons crossing the joint from the muscles above and below, and your patella (knee cap) floating on top. A grinding sound is often due to your quads muscles being too tight causing compression of your patella bone onto femur and tibia knee bones. Clicking, locking or giving way is usually a sign of torn meniscus (cartilage) inside your knee.
Keeping strong gluts (buttock muscles) will take a lot of stress off your knee joint. Putting your weight through your heels instead of your toes, when walking up stairs or hills, will help activate your gluts and help you get up easier. Biking is by far the best exercise you can do for your knee. It helps increase synovial fluid into the knee to clear away swelling and self-lubricate the joint. You could start by cycling your legs in a pool if you needed to.