Cartesian Diver Experiment (Video)

How do fish float and sink?

This classic experiment is called Cartesian Diver, named after French scientist, mathematician and philosopher scientist, René Descartes.

It demonstrates the principles of buoyancy and density and explains how fish dive or rise in water.

Buoyancy: the force of buoyancy on an object is equal to the weight of the water displaced by that object.

Density: if an object weighs more than the weight of the water displaced, it is more dense and will sink.

If it weighs less than the weight of the water displaced, it is less dense and will float.

Let’s use a pack of ketchup to show you how fish floats and sinks.

Cartesian Diver Experiment

Cartesian Diver Experiment / Diving Ketchup Packet

Prep Time: 30 minutes
Active Time: 30 minutes
Total Time: 1 hour

In this experiment, the packet of ketchup the fish.

We will make it sink and then float again.

You can make it go up and down on command. Isn't that amazing?


  • bowl
  • water
  • unopened packet of ketchup


  • bottle with an airtight cap (a screw cap is better than a flip one because it won't flip open under the pressure from the inside)
  • adult supervision



    Make sure that the ketchup packet you have will work in this experiment.

    Fill the bowl with water and drop the packet into it. If the packet floats, you have a winner. If not, try another one until you find one that floats.

After finding a good packet of ketchup to use, we can start the experiment.


  1. Fill the bottle completely with water.
  2. Put the packet of ketchup into the bottle. You may have to squeeze the packet a little bit to fit it into the bottle's opening. Be careful not to break open the packet.
  3. Top the bottle with water again before closing the cap.
  4. Now close the cap carefully. Try not to spill any water.
  5. Here is the fun part. Squeeze the bottle and see the ketchup packet sink to the bottom.

Did you try this project?

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Normally, the ketchup inside the packet is heavier than the weight of the displaced water, and therefore denser, which should make the packet sink.

But there is also some air in the packet. 

Since air weights a lot less than the same volume of water and is therefore a lot less dense, the whole packet’s overall weight/density is less than the weight/density of water and so it floats.

As you squeeze the bottle, you apply pressure on the bottle, which increases the pressure on the water, which in turn increases the pressure on the packet and causes the air to compress to a smaller volume.

Because the same amount of air now occupies a smaller volume, it becomes denser. 

The water is also compressed but not as much as the air. 

When the packet’s overall density is greater than the water surrounding it, the packet sinks. 

If you release the grip, the packet floats to the top again.

Most bony fish use swim bladders to dive, rise or maintain its position in the water.

The ability to do so is important because it allows the fish to conserve energy for feeding or hiding from predators.

A swim bladder is a balloon-like organ inside the body. 

It functions similarly to the air inside the ketchup packet.

But in addition to varying the size of the air pocket, a swim bladder can also exchange gases in water to change the amount of air inside. 

To reduce its overall density, a fish fills the bladder with oxygen from the surrounding water.

The bladder then has a greater volume, but its weight is not greatly increased.

When the fish is bigger, it displaces more water.

As a result, the fish experiences a greater force of buoyancy and it rises.

To increase its overall density, a fish releases carbon dioxide.

The bladder is then deflated and the fish sinks to the ocean floor.

To stay at a particular level, a fish fills its bladder with gases until the weight of the displaced water is the same as the fish’s weight. 

Then gravity and force of buoyancy cancel each other out, and the fish remains at that level.

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