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?




get a model submarine.



put a balloon in it, attach a hose to the balloon.



inflating and deflating the balloon will cause the submarine to rise and sink.



How do I make a submarine (for a school project - Gr.8) that has a change in it's volume?
the above answer is good you can attach a balloon inside and when its filled with water it will float and when its exh, it will sinks again but you have to assure that the submarine will sinks first when there is no air inside the balloon and also the amount of air inside could raise the submarine simply try and error :D