Submarines go up or down based on their buoyancy. That is when they weigh less than the water they displace they go up. When they weigh more they go down. If they weigh exactly the same they float right where they are. Submarines vary their weight by adjusting the amount of air in the ballast tanks. You can use a pen cap and a bit of modeling clay to build a submarine that goes up or down as the amount of air in it's ballast tank shrinks and enlarges. 
You need a pen cap, modeling clay, a plastic bottle with a mouth large enough to get your "submarine" through and the bottle cap. (The cap must close air tight on the bottle so that when you squeeze the bottle the water won't come out the top).
You need a pen cap, modeling clay, a plastic bottle with a mouth large enough to get your "submarine" through and the bottle cap. (The cap must close air tight on the bottle so that when you squeeze the bottle the water won't come out the top). - Make your submarine by putting a ball of clay on the stem of the pen cap. The opening to the hollow portion of the pen cap should be facing down. The hollow portion is your sub's ballast tank.
- Put the "submarine" in water (clay down so the air is trapped in the hollow portion of the cap) and then add or subtract clay until the "submarine" floats just below the surface of the water. It now has neutral buoyancy.
- Fill the bottle with water and put the "submarine" into it. The sub should float just below the neck of the bottle. If it falls to the bottom or floats on the surface adjust the amount of clay.
- Make sure the bottle is full and put the bottle cap on tightly.
- Squeeze the bottle. This will cause the pressure inside to go up and any gas trapped inside the bottle (like the air inside the pen cap) will shrink. This will change the buoyancy on your "submarine" from neutral to negative and it will sink to the bottom. When you release the pressure the air will expand and the sub will rise.
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