Cartesian Diver

Make a Cartesian Diver
  This a simple experiment that applies several interesting science concepts. It is suitable for children 6 and older but should me made with adult supervision.
Materials:

•   Fast food or Chinese food condiment packet

•   Empty plastic soda or juice bottle.

Procedure:

1. Clean and empty the soda or juice bottle.
2. Gently push the sealed condiment package into the empty bottle.
3. Fill the bottle almost to the very top with tap water.
4. Squeeze the bottle and watch the "diver" dive as you apply pressure. If the diver does not dive you can trouble shoot with the amount of water added to the bottle. It may take a bit more water than what you started with, or a bit less, but the water level should always be close to the top.  

The Science:
  The Cartesian diver is said to be named after the great explorer Rene Descartes who may have been the inventor of the toy. With no pressure on the bottle, the condiment package is buoyant meaning that there exists and upwards force from the water on the package keeping it afloat. The diver is not very dense meaning it has little mass per volume. When the bottle is squeezed, only the air in the bottle can be compressed (liquids cannot be compressed). The only air in the bottle is at the very top between the top of the water and the bottle's cap and a small amount of air in the condiment package. That small amount of air in the packet is the key! When the air in the condiment package is compressed, the package becomes less buoyant because it has the same mass but takes up less space (it is more dense). With less buoyancy the package can sink to the bottom of the bottle.

Extensions:Young scientists can . . .

1. explore with different types of condiment packages.
2. color the water (with adult supervision) or use water-proof markers to color the condiment package.
3.  maker a diver game by  bending a small piece of wire into a hook shape and taping it to the condiment package. Next, take another small piece of wire and shape it into to any shape you like, but include a loop of wire on the top. This "treasure" you have made must not be buoyant, but also must be light enough that it can be picked up by the hook on the condiment package. Now  the young scientist can squeeze the bottle lowering the hook and try to retrieve the sunken "treasure".
4. experiment with soda water filling the bottle.