4 Answers

you contain air, and air is less dense than water, therefore causing you to float. 

http://www.ehow.com/video_2366154_float-water.html

You displace more water then your body weight,therefore you are lighter and so float like and air bag.

Why some heavy bodies float and light weight objects often sink in water had long been a puzzle to mankind. The first person credited for discovering why this occurs and giving the world a formula for determining floatability was a Greek mathematician named Archimedes.

Legend tells us that Archimedes began with an experiment. He noticed that the water level was higher when pebbles were in an urn but without any pebbles the water level was lower, though the amount of water stayed the same. He then dropped various items into urns with identical amounts of water and carefully measured and weighed how much water was displaced by the objects. He learned that although a pomegranate weighed far more than one of his small pebbles, it floated, while the pebble sank to the bottom of the urn. He also discovered that rocks displaced more water than an equally heavy melon. The measure of water displaced by the fruit was less than what was displaced by the same weight of rocks!

Archimedes realized that at last he had a means for measuring something other than weight. We call this measurement Specific Gravity. Specific Gravity measures how closely packed or condensed the molecules and atoms are in a body. The ability to float depends, therefore, on how much space is between a body's smallest components.

It's particularly important for Water-Shy people to understand the technical reasons why our bodies are able to float in water. Knowing WHY we float transfers into BELIEVING we will float. This knowledge may remove anxiety or panic and possibly prevent a drowning.

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To find the Specific Gravity of an object in water we divide the weight of the object by the weight of the displaced water. Water is given a Specific Gravity of "one." If a person weighs 140 pounds and displaces only a pound of water her Specific Gravity is 1.4. If the displaced water weighs five pounds, her Specific Gravity is 2.8.

If an object's Specific Gravity is over "one" it will begin to sink; if less; it will float. With lungs fully inflated and a moderate amount of fat, most people have a Specific Gravity from two to four.

If relaxed, not moving and in a vertical position our bodies will only sink a foot or two below the surface. Those with a greater proportion of bone and muscle which are denser than fat, will sink further below the surface. Their greater muscle strength while treading water, however, may easily compensate for less floatability as they are able to keep their heads above water with less effort.

Specific Gravity is decreased when an object's weight is dispersed over a larger area. For example, a person lying atop the water with arms and legs spread apart will more readily float than if they are in a verical position.

MUSCULAR DEVELOPMENT: Muscles are composed of elongated red and white cells. Generally, most adult men have more muscle cells than adult women. The NUMBER of muscle cells in our bodies doesn't increase, but the cells DO increase in SIZE as we exercise. The larger they are the more firmly packed they become within the muscle wall. This increases the body's Specific Gravity. When we tense our muscles they become even more condensed causing a higher Specific Gravity. It's important to know that the tighter we hold our muscles the less we will float. Remember: A RELAXED BODY IS A FLOATING BODY!

That is easy to say but when fear or panic is foremost, it's not easy to relax, though not impossible. As shown in a previous chapter, relaxing the frontal muscles and breathing deeply could save one's life.