I've been trying to get in better shape so I've been running on the treadmill. While bored running on the treadmill, it occurred to me that Newton's third law applies to running! I always thought my knees hurting while running was caused by having bad knees but really it's the force of the treadmill back on me! As I put force on the treadmill the treadmill puts a force back on me. However the treadmill has a much larger mass than I do therefore it hurts my knees!