Be kind to your knees, you'll miss them when they're gone

Too much biking could make your knees unhappy, especially if your bike is not well adjusted, or if you perhaps have six gears and go over big hills anyway. So, what type of pain is it? Did you fall? Should you readjust your bike? Learn how to ride better? I'm going with the last option, because I'm not getting more gears any time soon.