Ask A Scientist

Why does your hand go through water, but you can float on top of it? 

Asked by: Anna Regulski
School: Johnson City Intermediate School
Grade: 3
Teacher: Mrs. Gorleski
Hobbies/Interests: Dancing, singing
Career Interest: Artist, fisherwoman, dancer, singer

 

Answer from Michael Little

Distinguished Professor of Anthropology

Research area: Human adaptation to the environment Interests/hobbies: Swimming, choral singing, antique toys and books

Well, Anna, this is an interesting question. First, your hand can be submerged in water easily because you can push it into the water with your arm. But it is easily submerged also because it has a density that is close to water. The term density means how much something weighs for the amount of volume it has. For example, most wood has a density less than water, so it floats, while a block of most metals will sink because metal (iron or lead or gold) has much greater density than water. That is, a block of wood feels light compared to a block of metal of equal size.

Now comparing the hand with the rest of the body is a different story. Our body is made up of a lot of different tissues, and a lot of water. Some of the tissues have a density that is less than water and some of our tissues have a density that is slightly greater than water. If these tissues could be separated and placed in water, then the heavy ones would sink and the lighter ones would float.

Some of our heavier tissues are bone and muscle, and one of our lighter tissues is body fat, or adipose tissue. Body fat is a very important tissue in the body. It stores energy from the food we eat and can help us survive during periods where we need extra energy. It helps pregnant mothers who need extra energy. It also helps to cushion parts of the body, especially when we sit down, but also if we fall down. For an average girl your age, she should have anywhere from 15 to 20 percent of her total body weight as fat. For an eight-year-old boy, it will be a little less. It is this body fat that buoys you up and allows you to float in the water, since the density is so low. If you have a lot of body fat, it may be difficult to sink and dive to the bottom of a pool.

The other thing that helps you to float in the water is the air in your lungs. You can run a little experiment. Lie on your back in a pool or lake and watch how your body sinks a little bit when you breathe out and rises when you take a deep breath. If you were to go to the shore to swim in the ocean, you would find that you floated even better than in a swimming pool, because the salty sea water has a greater density than fresh water. If you were to go to Israel and to swim in the Dead Sea, you would float very close to the surface because the density of the water from lots of salt is extremely high! Our body fat helps us to swim by keeping us afloat. Be happy that you have body fat because it is important for our health. Just don’t have too much of it, because too much can hurt us and lead to poor health.

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Last Updated: 9/25/14