Your knees just want a little support. The best thing you can do to keep your knees healthy is to strengthen all the muscles around them. And one of the best ways to do that is your basic front and back scales. They're incredible for building stability throughout the lower body and the core, and they are no-impact, all of which is great for your knees.
They look simple. But damn. Try doing a full, slow, smooth set of front and back scales and see how easy they really are(n't).
If you want to take your leg strengthening even further, we have this article. It includes 9 different lower body exercises designed to increase strength, mobility and balance. Another great addition to improving knee stability.
Be sure to check out our knee article for more advice on improving and maintaining overall knee health.