My knees hurt when i exercise?!

Well the funny thing is that you do indeed have strings, "hamstrings" that can indeed rip, and mind you will hurt horribly. These are a group of muscles that actually used to flex your legs at the knee. So if you are over extending you may strain, and even "pull" these. I actually would suggest that you work towards being able to stretch that far, you can do so by doing a semi-toe touch, and sitting with legs apart and reaching to each side for your toes, DO NOT try to do it in a day, if you are unable to touch don't get upset continue to stretch these muscles and eventually you will be able to do so with ease.

Now as for your knees hurting, you say they crack? Do you actually hear a crack in your knee during regular exercise, or is this only happening when you are exercising on your knee caps? If you are hearing it during normal exercise then perhaps you should check with your doctor about possible fluid on your knees, if it only happens when you are on your knee caps then the surface you are exercising on is too hard and I would suggest getting some type of foam pads, They sell some that puzzle together so you can form an area for you to exercise on without fear of stepping off if you do cardio/yoga things that may cause you to need to stand and kneel.

I'm not a doctor so of course if this is causing you distress etc, then check with your physician. I hope this helps you out. :)

just do regular stretches and exercises if things do not get better pop to the Doc's x