Question: In a swimming pool, and I try to float on my back, why do my knees an legs sink?

Claire Lee answered on 27 Jun 2013:

I think it’s because your body is filled with air – in your lungs, so it has a natural buoyancy, whereas your legs are more solid so they are denser and sink.

You get positive buoyancy (meaning you float), negative buoyancy (meaning you sink) and neutral buoyancy (meaning when you are completely submerged in water you don’t go up or down).

Quite a fun thing to do is to lie in the water and breathe in and out, and watch how your buoyancy changes – you should float quite well if you keep your lungs full of air, but breathe out completely and you should start sinking 🙂

Also, your body’s natural buoyancy is dependant on what it’s made of. Since fat is less dense than muscle, a fatter person will float more easily than a lean person.