Re: Why can human beings float in water?

Any material whose density is lower than that of water will float on the surface 
of water; if the density is higher than that of water, it will sink. The specific 
gravity is the weight of a sample of material relative to the same volume of 
water.
Styrofoam has a specific gravity around 0.05: It floats with about 95% of it 
above the water surface and 5% below. Wood and ice are about 0.9: 10% above the 
water and 90% below. Most rocks have specific gravity around 3 to 3.5, iron is 
around 8, lead around 11.5, and gold around 19.

A person is made up of a number of different materials. Fat has specific gravity 
around 0.9, muscle around 1.3, and bone around 1.8, and the air in our lungs 
close to zero. The overall result comes out that on average a person has a 
specific gravity very close to 1, but usually slightly below. A fat person floats 
better than a thin person, and any person floats better with a good lungful of 
air. When a person floats, as little as 1% may be out of the water! The trick is 
to make sure that this 1% includes your nose and/or mouth! And of course floating 
is helped enormously if you give yourself a little extra help up with gentle 
motions of your hands and/or feet.