Sink or Swim

Airplanes and sharks have two things in common. They have the same shape, and if either one of them stops moving, they start to sink. Most sharks have to keep moving or swimming to get oxygen from the water, but a few sharks have found other ways.

The nurse shark has special muscles around its gills that constantly move the water over the gills so that it can stop and rest. The sand tiger shark takes a break by floating, without swimming, never dropping completely to the bottom of the water. To do this, it simply swims to the surface of the ocean, takes a big swallow of air, and then sinks slowly down into the water -- over and over again. Probably some of the people who have seen the sand tiger shark doing this have thought they were looking at a whale.

Humans have developed many inventions like boats, rafts, and life-vests to make sure that they don't sink in the water. Can you think of other things we use to protect ourselves in the water?

Going Under

Do you sometimes wish that you could breathe under water without a snorkel mask? Let's do some tests to see how you and your friends' underwater breathing skills would compare to a shark's:

  1. Take a deep breath and see how long you can hold it.

  2. Now see how long your friends can hold their breath.

  3. Now, measure around your chests. Were the people with the biggest chests the ones who could hold their breath for the longest time?

  4. Try measuring your chests before you take a breath and again while each of you is holding it.

  5. Compare the measurements. Who took the deepest breath?

When you visit the doctor, ask if you can listen to your lungs with the stethoscope while you're taking a deep breath.

In the ocean, sharks seem to be swallowing water, but actually they are running the water over their gills, which take in water like a sponge. The oxygen in the water travels through the gills, making a trip through the shark's blood and then back out again to be released through the sponge-like gills. This gets rid of the other part of the air that the shark's body doesn't need.

Why didn't the shark buy an electric toothbrush?

Use the tooth decoder to figure out the answer to this riddle. Write the letters on the lines.

Words to Know


When something is said to have buoyancy, it means that it is able to float. Some sharks use their buoyancy to rest on the top of the water in the sunshine.

Staying Afloat

Most sharks don't sink because they are constantly on the move. They also have natural buoyancy because the cartilage in their skeleton is much lighter than your bones.

Buoyancy is the quality that determines how well a shark or an object can float in the water. A log will float for a long time until it becomes completely soaked with water, and then it sinks. The first boatbuilders might have seen a log floating down the river and thought that riding, instead of walking, was a better way to travel.

They probably thought it was easier to ride in the log, instead of on top of it, and started to hollow it out. Do you think they kept putting more of their friends into their boat, until it sank? If you were a boat builder, how would you decide how much weight your boats can hold?

Ask your parents if you can try a few experiments to see how well things float:

  • Try placing a dry sponge in the bath water. Does it float better right away, or after it has been in the water for a while?

  • Place several of your toy boats in the water. Why do you think some of them float so much better than the others?

  • Put some water in the boats. How much water does it take to sink them?

What's worse than seeing a shark's fin when you are swimming?

Connect the dots from 1 to 73. Then break the number code and fill in the boxes!

Experiments in the Kitchen

Icebergs are just great big ice cubes made from compressed snow, which is basically plain water. So why do they float so well in the salty ocean? If you lived near the Great Salt Lake in the United States, you would see how easy it is to float in its water because it is so salty. Ask your parents if you can use the freezer for a little while to compare how well things float in plain or salty water:

Try This: Underwater

Have you ever tried to swim down under the water or sit on the bottom of the pool? Your body naturally floats up toward the surface because you have air in your lungs. Try blowing a little air out while you are under-water at the pool or in your tub. Does this help you stay down for a longer period of time?

  • Try adding a teaspoon of salt to 1½ cups of water and freeze it in an ice cube tray. Be sure to remember to throw these cubes out after you're done with the experiment!

  • Compare which cubes float best in a glass of plain water: the salt cubes or the ice cubes.

  • Now put a teaspoon of salt in a glass of water and stir it up.

  • Try floating the different types of ice cubes in the salt water. What did you find out?

Floating Treats

Do some frozen liquids float better than others? Try freezing several different types of fruit juices or soft drinks in plastic ice cube trays. Do those cubes float as well in the water as the ice cubes? Now pour some other liquids in glasses and add the juice cubes to them. Do the juice cubes float better in a glass of juice or soda? See if your friends can guess what the original flavors of each of the juices were. Maybe you will have created a new flavor! Keep trying different combinations of liquids and juice cubes until you find one you really like.

Ice cream also floats on top of soda or fruit juice. Try more buoyancy experiments with banana slices, grapes, cherries, marshmallows, pineapple slices, strawberries, and mandarin oranges.

Is Oil Lighter Than Water?

The biggest fish in the ocean is the whale shark. As heavy as the whale sharks and basking sharks are, both of these huge creatures remain close to the top of the water. That's because the oil in their liver lifts them up to the surface. A bony fish like the goldfish has an air bladder to keep him from sinking. Will an oil-filled balloon float better than one that is filled with water or air? If you want to see if oil really does help the shark float, you will need three small balloons (all the same size), three rubber bands, a small funnel, and some cooking oil.

Which One?

I may be one of the friendliest and smartest things in the sea. You can see me perform in water shows. Do you know which one I am?

  • Dolphin

  • Killer whale

  • Seal

  • Penguin


  • Blow up the first balloon with air. Fasten it with a rubber band.

  • Fill the second balloon with water from your faucet and fasten it, too.

  • For the last balloon, ask an adult to help you pour cooking oil into it, using the funnel. Then fasten it with a tight rubber band.

  • When you are through filling them, put all three balloons in a bathtub half-filled with water.

Which balloon floats the best? Would you rather be the shark filled with oil or a goldfish filled with air? Some kinds of sharks float so well in the water that they can ride the warm currents rising through the water, like a glider does in the air.

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