Why do you float when you lay on your back in the water?

Answer 1

This happens because your body is less dense than water. It is the very same reason why a helium filled balloon will float upward in air. Helium is less dense than air, and so the more dense air settles under the balloon and moves it up; the balloon is 'seeking' the surface of the air mass that it is floating in.

Another way of thinking about this: If you could measure out the amount of water that weighs exactly the same as your body weight, you will find that the volume of that water is less than the volume of your body. Same weight in a smaller volume-- that means the water is more dense than your body. The density of water is really called specific gravity, but the concept is the same.

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NOTE: This turns out to be very useful as a way of measuring over-all body fat, or the Body Mass Index. Some people can be standing in 3 or 4 feet of water, exhale the air from their lungs as completely as possible, and have time to float down to the bottom of the pool and lay there for a moment before coming up for air. Others cannot do this, no matter how hard they exhale. The difference is the amount of body fat. body fat is less dense than muscle, so it works to keep the body at or near the surface.