Key concepts
Archimedes’ principle

Have you ever thought about why most fishes never sink to the bottom of the ocean or float to the water’s surface? How is it that they can stay so perfectly buoyant underwater? You might be surprised to hear most bony fishes have a special organ to help them with that: a swim bladder. This is a thin-walled sac located inside the body of a fish that is usually filled with gas. Besides helping fishes stay buoyant it can also function as a sound producer and receptor or as an accessory respiratory organ. Wonder how a swim bladder works? Try this activity to find out!

You probably know objects can sink or float when placed into water. In both cases the water moves out of the way, or is displaced, to make space for the object. An ancient Greek scientist named Archimedes discovered this principle of buoyancy. In physics buoyancy is an “upward force that pushes on an object that is immersed in a liquid.” If you have ever tried to push a beach ball underwater, you have felt this buoyancy force in action. The ball seems to push back up although you are pushing it down! Archimedes also discovered the strength of the upward acting force exerted by the liquid is equal to the amount of water displaced by an object. At the same time the volume of displaced water is always the same as the volume of a submerged object. This is why the water-displacement method is a great way to determine the volume of an irregularly shaped object.

Whether an object sinks or floats is dependent on which force is larger: the force of gravity pulling down on the object (its weight) or the buoyancy force pushing up on it. If the buoyancy force is larger than the object’s weight, it will float on the water’s surface. If the object’s weight exceeds the buoyancy force, however, it will sink to the bottom. When the buoyancy force is exactly the same as the object’s weight, the object has neutral buoyancy and remains at its level. This means the object’s density becomes important as well. Dense objects have a high mass and only displace a low volume of water; they have a large mass-to-volume ratio. If an object has a small mass but displaces a lot of water, it is less dense and therefore has a low mass-to-volume ratio. Objects denser than water will sink whereas those less dense than water will float.

What does all this have to do with a swim bladder? For a fish to be buoyant, or float, it must displace an equal or greater amount of water than its own body mass. The trick is the swim bladder, which is basically like an air-inflated balloon that can expand and contract depending on how much gas is inside. When the swim bladder expands it will increase in volume and therefore displace more water. This increases the fish’s buoyancy and it will float upward. When the swim bladder deflates the fish’s buoyancy decreases and it will sink as it displaces less water. Divers use the same concept for their buoyancy-control devices.


  • Small glass bottle
  • Balloon that fits inside the bottle
  • Plastic tubing (at least 30 centimeters or 12 inches long)
  • Big tub or container that is waterproof and preferably transparent (It should have enough room so the glass bottle can float in it freely.)
  • Strong, waterproof tape
  • Water
  • Towels


  • Find a workspace that can withstand water spills.
  • Push one end of the tube through the opening of the balloon, and tape the balloon and the tube together, creating a seal. After that you should be able to inflate the balloon by blowing through the tube. Make sure the connection between the balloon and tube is airtight.
  • Put the balloon inside the glass bottle and tape the tube to the bottle opening.
  • Fill the tub with water.


  • Place the bottle with the balloon inside into the tub of water. Keep the other end of the tube outside of the tub. What happens when you place the bottle into the water? Does it float or sink? Why?
  • Carefully blow through the tube so the balloon inside the bottle inflates a little bit. Does inflating the balloon a little bit change the floating behavior of the bottle?
  • Inflate the balloon even more until it almost fills the whole bottle. What happens as you inflate the balloon more and more? Can you explain your observations?
  • Next, let the balloon deflate. How does that change the position of the bottle in the water?
  • Play around with inflating and deflating the balloon inside the bottle. Can you find a correlation between the balloon size inside the bottle and the latter’s floating behavior?
  • Extra: Try simulating different sizes of fishes by using different types of bottles or balloons. Do all of them give you the same results? What if you put a tiny balloon in a very big bottle or vice versa?
  • Extra: Does the activity also work with a plastic bottle instead of a glass one? Why or why not? Test it to find out!

Observations and results
Can you see how what you built resembles a fish? The glass bottle mimics the fish’s body, and the balloon represents the swim bladder inside its body. When you put the bottle into the tub, it should have filled with water and sunk to the bottom. This is because the weight of the glass bottle filled with water is larger than the buoyancy force pushing it up. This changes when you inflate the balloon; when it expands it pushes the water out of the bottle. You now have an air-filled balloon inside the bottle. Because air is much lighter than water, the bottle’s weight lessens as the balloon fills with more air. The bottle eventually becomes lighter and thereby exerts a weaker downward push than the buoyancy force pushing the bottle up. At this point the bottle will start floating upward toward the water's surface.

When you deflate the balloon again the reverse happens—the bottle becomes heavier due to the water flowing back into the bottle, and it starts to sink. You might have noticed the activity doesn’t work as well with a plastic bottle. This is because plastic itself is less dense than water and tends to float whereas glass is denser than water and therefore sinks.

Detach the tubing from the bottle and the balloon and dry all water spills and materials with a towel or paper towels.

More to explore
Buoyancy, from HyperPhysics
How Do Fish Rise and Sink in the Water?, from How Stuff Works
Buoyant Science: How Metal “Boats” Float, from Scientific American
Science Activity for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies