How do I make a submarine (for a school project - Gr.8) that has a change in it's volume?

My daughter has a (Gr.8) project - she needs to make a submarine which Floats then sinks and comes back up but it cant be a change in its weight - it has to be a change in its volume. She can not touch the submarine itself although she can touch a tube if their is one attached...PLEASE HELP!

How do I make a submarine (for a school project - Gr.8) that has a change in it's volume?
What do you mean? Why not just push it down if she can touch the tube?How do I make a submarine (for a school project - Gr.8) that has a change in it's volume?
For the submarine, find a tiny container, one that is able to trap air and fits through the opening of a CLEAR 2 liter bottle. Plastic shell of a ball point pen should do nicely.



Seal one end of the tiny container - with silly putty and / or tape - so air could not escape, leaving the other end open. Wind the outside of the opened end with wire - small portion of a coat hanger should do. Secure the weight so it does not come lose.



The wire should weight enough to keep the open end from surfacing and it should be enough to ALMOST sink the tiny container with air still trapped inside it. The submarine is now completed and ready for a test.



Fill up the clear 2 lite bottle. It does not have to be air free, but it should have plenty of room for the submarine to move up and down. Drop the submarine into the bottle. Open end down to trap air in it. Close the bottle cap, make sure the seal is both air tight and water tight.



Squeezing the 2 liter bottle hard should sink the submarine - as the volume of air is compressed by the pressure of the water.



Releasing the 2 liter bottle should release the pressure of the air and volume of it would increase and the submarine should once again surface to the top.



NOTE that this submarine is vertically oriented.



This thing is simple enough and does not involve mechanical tinkering of diaphragm, pump, tubing ... etc. It's limitation is that it work only within the 2 liter bottle as the bottle is part of the whole system that makes it work. The most critical part is the weight of the wire used.