Right, for this one you'll need:
A large empty pop bottle with lid
The top from a bic pen
Fill the pop bottle to the top with water, place to one side. To make your 'submarine' close the little hole in the pen lid with a small blob of sticky tack then make a ball and push the lid's 'stick' into it (this is the balast). Drop the lid into the bottle - large tack-blob first so that an air-bubble is captured inside the lid. If the lid sinks, reduce the amount of Sticky tack, if it doesn't float just under the surface add a tiny bit more. When you're happy replace the bottle cap. Then comes the fun part - gently squeeze the bottle to make the 'submarine' sink to the bottom and release to let it float back to the top! With practice you should be able to 'hover' it in the middle.
The science bit.....:teacher:
When you squeeze the bottle, the bubble is also squeezed, making it smaller and allowing more water inside the pen lid - so it sinks. Releasing your grip allows to bubble to re-expand, making it lighter and able to float!
Also posted in 'Science and Processes'