My left knee hurts

Without knowing more, it could be the tendon that connects the outside of your leg to your hip (the iliotibial band). It's a very common thing in runners. What you need to do is to stretch as much as possible, especially this band. Check out this website for helpful treatments: http://www.time-to-run.com/injuries/thebig5/itb.htm

what is your age.... if you are at age when you are growing upp...some parts of your body can hurts...and... i dont know...baybe you should go to see a doctor...it is better that ....