This Instructable will show you how to create your very own, pointless, gratifying and scientific wonder-toy. I will show you what I did, how to alter it for yourself, and explain the science behind it so you can apply the theory to other projects!

A Cartesian diver is an experiment (or toy :D) designed by Descartes in the 17th Century. Basically, you have what looks to be a bottle of water, and if you gift it you will be able to see the disappointment on the receiver's face. But then when the bottle is squeezed, a magnificent object will float down, creating an amazing sense of excitement between the person who sees it for the first time and yourself, who has been playing with it for three days!

I apologise for the quality of the pictures - my camera decided to go on holiday for a bit. However, if anybody else makes one, I would love to see them, and if possible, include their creations and useful experiences on here!

Step 1: Materials

Things you will need:

Bottle (Glass or Plastic - I'll show how to make either one work)
Water (preferably iodized)
An object that barely floats in water
A world where the natural laws are the same as those that physics textbooks suggest.
Balloons, rubber bands (or the necks of other balloons) if you are using a glass bottle

If you're making it for fun and not for looks, floating objects could include:

a sachet of condiment (ketchup, mayonnaise etc)
a closed pen lid with some blu-tack on the bottom
the plastic wrapper of something closed into a balloon with tack or similar.
a pipette - fill with enough water so that it just floats

Step 2: Making the Diver

You can shell out for a professionally made glass diver, which has a hole in its back to allow some water to get in for weight. The tail often winds around the body so that when the pressure is released and the diver floats again, he spins as he rises.

For mine, I cut the sides off of a rubber bouncy ball so that it became a disc shape, then carved it into a heart. I tested this in a glass of water and it floated no problem, so I needed to add a bit of weight.

I then got a felt-tip pen lid of a similar colour and cut it down into a tiny bucket, and attached the two with blu-tack and the shaft of a cotton bud, making a kind of 'heart air-balloon'. After a long time removing air bubbles in the blue tack by pushing it down with tweezers, and adding/removing miniscule amounts of blue tack to make it heavy enough, I finally achieved a workable object. However, I found the blu-tack coming apart in the water and it stopped working, so I replaced most of it with hot glue, and it works fine.

When making your own, it is important to keep testing it in a glass of water, so you can tell if it floats too easily or sinks. Once you think you have a working object, try it in a plastic water bottle, as then it is easily removable and testable (you'll see why testing with a glass bottle should be left until the end in the next step). It is also quite important to make sure your diver fits through the neck of the bottle! If you are confident enough, you can construct it inside the bottle, but you'll need a wide-enough neck to fit your tools through.

I recommend leaving it for a few days and playing with it to make sure it works over time!

Step 3: Preparing Your Bottle

For plastic bottles:
Some water bottles have very small necks, so they might not be the best to use. Oasis bottles have quite a large neck, and are ideal for bigger divers, but most bottles will work easily, and can be coaxed into allowing larger item through by bending the plastic with pliers or something.

Remove the label - most labels on plastic bottles come off quite easily. If there is and glue residue left on the bottle, soak it off in warm, soapy water, or use a rough sponge. Make sure the bottle is clean inside and out, and if you want to make it extra special, then add your own label.

Fill to the top with water and insert your diver (some water should be displaced). Screw the cap on tightly, and you're ready to go.

For glass bottles:
Wine bottles aren't the best, but they'll do if your diver is quite small. Glass bottles with larger necks can be harder to come across, but I think some glass bottled soft drinks come in smaller bottles with wider necks.

Remove the label by soaking in warm soap water for a while until it slides off, and clean any glue off with a cloth or rough sponge. Add you own label if desired (like I did), and if you want extra points you can base it on a wine label, or something. Make sure it isn't too big or long to obscure the view of the diver, though!

Fill to the top with water, carefully insert your diver (after many tests in a water bottle - if he sinks you'll have a job of getting him back out, which will lose you quite a bit of water). Once he's inside floating happily, cover the top with something that will allow you to apply pressure to the water inside. I used rubber balloons, and used quite a few, and the necks of other balloons, to make sure it is watertight, so that when I apply pressure, the water doesn't escape down the outside of the neck. You could also use the rubber squeezer thing on pipettes, just make sure it is on tightly enough. Rubber bands and string may also be helpful in making it water-tight.

The label I made is based on the book 'The Midwich Cuckoos' by John Wyndham. It is a thoroughly recommended read, although will not help you with this project. It is nice to have a theme to work with, though! Even if is just wine - you could use a modified cork as a diver!

Step 4: Using the Cartesian Diver

This is the most fun, and also the simplest part of the whole process, if you've done it right. Otherwise, this is the most annoying, frustrating and apocalyptic part (so be careful if you've used a glass bottle).

If using a plastic bottle, squeeze the sides slightly to increase the pressure.
If using a glass bottle, press on the balloons through the neck-hole/ squeeze the pipette to increase pressure.

It 'should' sink!

You can vary the pressure to vary the speed in which it sinks/floats, or even apply just enough to get it to stay still in the middle (maybe you could secure it in this manner so that you have a floating object in the middle permanently?).

Sometimes there may be some air in the top of the bottle that you cannot see, and the diver will not sink without greater pressure than you'd expect, or will take greater pressure than it used to. If this happens, just sort of swirl/shake the bottle around a bit, and you should be good to go again.

I made a mistake with the blue tack as it held tiny bits of air and started to come loose and float in the water, and my diver then refused to work properly, so I replaced the blue tack with hot glue.

At this point, you should feel like some kind of deity. You control the fate of the diver, at the touch of a finger!

Step 5: The Science

It's all to do with buoyancy. The diver will contain two substances, water and air. In my diver, for example, the air is trapped in the heart, and the water in the bucket, and in a few tiny pockets on the heart. When pressure is applied, it will affect the least dense material (as it would take less pressure to enact the same reduction that it would need for the most dense material), which is air.

As the air pocket(s) becomes smaller, there is less buoyancy as there is now more water than air, and so it sinks. When pressure is lifter, the air expands again, giving the diver more buoyancy as the amount of water displaced is now the same as it was.

More information:
http://en.wikipedia.org/wiki/Cartesian_diver
http://en.wikipedia.org/wiki/Buoyancy#Archimedes.27_principle
http://en.wikipedia.org/wiki/Ideal_gas_law