'Squidy' the Diver

What you need

• Glass eye-dropper
• Clear plastic bottle
• Cup or glass of water
• Water (to fill bottle)

What to do

1. Fill a glass or cup with water.
2. Fill the eye-dropper ½ - ¾ full with water from the cup, leaving room for a small air bubble.
3. Place the eye-dropper in the glass of water to test that it floats. It should just barely float with only a small amount of the top exposed above the water line. If your eye-dropper is floating too high, add more water to it. If your eye-dropper sinks below the water line, take some water out.
4. Fill a clear plastic bottle with water.
5. Place the eye-dropper inside.
6. Add some more water so the bottle is filled right to the brim and then screw the cap on.
7. Squeeze the sides of the bottle and watch ‘Squidy’ sink.
8. Release the sides of the bottle and watch ‘Squidy’ rise to the top.

Note: Try not to turn the bottle upside down as the air bubble might escape and ‘Squidy’ will not be able to rise again. If this happens, take the eye-dropper out of the bottle and simply squeeze some of the water out before placing it back in the bottle.

Extension

Decorate the eye-dropper to look like a squid. Draw some eyes onto the top of the dropper. Add some tentacles around the bottom of the dropper - attach the neck of a balloon, cut off the round part of the balloon and then cut the remaining rubber into thin strips to look like tentacles.

How it works

A Cartesian Diver works in a similar manner to how a submarine dives and rises. When the bottle is squeezed the air bubble inside the eye-dropper gets compressed or squashed, allowing more water to enter the eye-dropper. The increased amount of water makes the eye-dropper heavier so it sinks. When the sides of the bottle are released, so is the pressure acting on the air inside the eye-dropper and the air bubble expands. As the air bubble expands it pushes out some of the water. This makes the eye-dropper lighter so it floats to the top of the bottle.

What to ask

• How can you make ‘Squidy’ sink?
• How can you make ‘Squidy’ rise to the top of the bottle?
• When the sides of the bottle are squeezed what happens to the water level inside the eye-dropper?
• Can you make ‘Squidy’ stay in the middle of the bottle?

Resources

Glass eye-droppers can be purchased from chemists. Make sure the eye-dropper will fit inside the opening of the bottle before purchasing. The squid in the photo uses a pipette, hex-nut and squid fishing lure. Alternatively, there are many other ways to make a Cartesian Diver that can be found on the web. Another popular method uses a flexible drinking straw and a paperclip or pipe cleaner.