I think I am going to start swimming for exercise. I used to really enjoy it when I was younger.
That's a good idea. Since swimming is the self-propulsion of someone through water, it burns a lot of calories.
That's good to know because I need to burn A LOT of calories. I need something more low impact for my knees too.
The elliptical trainer is also good for low impact. It doesn't cause excessive pressure on the joints.
That's a good idea.  I wonder what swimming stroke would be the best for exercise. 
I'm not sure but there are so many different strokes I"m sure you could figure out which one.
How do we not even sink when we swim? It's fascinating to me.
When you swim you use locomotion. It's achieved by the coordinated movement of arms and legs.