My knees hurt when I stretch. Should I still do it?

First, you should discuss your knee symptoms with your physician and follow his or her recommendations. In the meantime, you might do the Exercises with reduced weight (maybe even use no weight) and through a reduced range of motion—whatever it takes for you to do the exercises without pain. Don't worry about the noises, but do avoid pain. Then progress slowly, cautiously increasing both the range of motion and the amount of weight you're lifting. Over time, you should be able to strengthen your legs and improve your flexibility.