Cartesian Diver

Kids can make a homemade diving toy sink or float on command with just a slight squeeze of the hand.
Supplies you’ll need:
Plastic water or soda bottle
Plastic straw that bends
Modeling clay
Warm or room temperature tap water
Funnel

Step 1
Cut straw about 1.5 inches on either side of the bend.

Step 2
Fold straw at bend to make u shape. The ends of the straw should be equal.

Step 3
Make your diver. Add a glob of modeling clay to ends of straw. Make sure the holes of the straw are sealed. You’ll want to add just enough modeling clay to make the straw buoyant. Too much clay and it’ll sink right away. Too little and the diver won’t dive.


Step 4
Test the diver’s buoyancy. This is the most important step. Place your diver in a large bowl of water. You want the diver floating near the top with the majority of it submerged. If it sinks to the bottom, remove some of the clay. If the diver floats too high above the surface, add clay. *The yellow straw is too far above the surface, the purple straw on the bottom sank as soon as it hit the water. The purple straw floating on the left is just about perfect.

Step 5
Once the diver has the proper buoyancy. Fill the plastic bottle with water. Make sure it’s the same temperature as your test water. We found it best to just use the test water from step 4. Fill it all the way up.

Step 6
Add your diver. Cap the bottle.

Step 7
Now squeeze. As you apply pressure to the bottle, your diver should sink. When you release the bottle, the diver will rise back to the top. If your diver doesn’t work: 1. Check to make sure the ends of the straw are plugged. 2. Make sure there are no holes in the straw. 3. Try using warmer water.


Step 8 (optional)
Decorate your bottle and play, play, play!

The science behind the toy:
This classic science experiment was named by scientist, philosopher and mathematician, René Descartes. It demonstrates the principle of buoyancy. An object is buoyant if the weight of the water it displaces exceeds the weight of the object itself.
Here’s what happens:
The air trapped inside the straw reacts to the changes in pressure. When you squeeze the bottle, you apply pressure to the air inside the straw which reduces the straw’s size. The straw now displaces less water, which reduces its buoyancy. That’s why it sinks. When you release the pressure, the air expands, making the straw displace more water so it rises back up.
How this applies in the real world?
Fish are able to rise or sink in the water by squeezing or releasing their muscles around a small air sac in their body. Submarines also work in a similar way. Water is pumped in and out of tanks inside the sub to make it rise and fall.