How to Make a Water Submarine That Floats, Then Sinks, Then Floats

Cut a straw using a knife, so you have a 3 inch length of straw. This is your replica submarine.

Put a lump of reusable putty adhesive over each end of the straw so you cover the holes. Drop the straw submarine into a glass of water. The straw sinks if you have put enough reusable putty adhesive on the ends. If it doesn̵7;t sink, add more reusable putty adhesive to the ends of the straw until it sinks readily.

Remove the straw from the glass and remove an equal amount of reusable putty adhesive from each end. Only remove a small amount. Put the straw back into the glass. If it sinks straightaway, then remove a little more reusable putty adhesive. What you need to achieve is the straw just beginning to sink, then floating back to the surface. You don̵7;t want it to completely sink, nor do you want it to readily float. The buoyancy of the straw submarine must be correct before proceeding. It may take a little time to get it correct.

Fill a 2-liter soda bottle with water nearly to the top. Drop in the straw submarine and then top up the bottle, so it̵7;s full of water. Put the cap on the bottle and tighten using your fingers. You see the straw submarine floating near the top of the water.

Squeeze the bottle using your hands. As you apply pressure, you see the straw submarine start to sink. The harder you squeeze the bottle, the deeper the straw sinks.

Release the pressure and the straw rises to the surface again. You can make the straw float, sink or stay suspended in the water bottle by increasing or decreasing how hard you̵7;re squeezing the bottle. The reason you can control the straw submarine is because when you squeeze the bottle, you are increasing the air pressure inside the straw and it becomes denser, so it starts to sink.