response: It's low-impact because the human body is only slightly less dense than water, which allows the water to support a person's weight
history: I love to swim and it keeps me in shape Nice!  It's a good "low-impact" sport, compared to running. Yes and I have lung disease its very easy for me to exercise in water, much easier than a gym workout Oh I see!  And swimming is also useful for survival, right? Yes and its also low impact on my bones and joints. Older people really benefit from swimming
knowledge: Since the human body is only slightly less dense than water, water supports the weight of the body during swimming.