Stuff You'll Need

• pen cap
• paper clip
• tape, aluminum foil
• plastic bottle with a cap
• water

How to do it

1. To start, stuff a little aluminum foil into the pen cap so that the aluminum foil plugs up the little hole in the cap. If you use too much foil the sub won't float - so just use a tiny piece.
2. Open the paper clip so that it has a hook on the end.
3. Tape the straight end of the clip to the stem of the pen cap.
4. To test your submarine, drop the pen cap in a cup of water.
5. The pen cap should float with the paper clip pointing at the bottom of the cup.
6. If it doesn't float, try adjusting the paper clip on the pen cap by moving it up and down the stem.
7. Once you get it to float, put your sub into a plastic bottle filled with water and screw on the cap.
8. Now squeeze the bottle. The pen cap should sink to the bottom!
9. When you squeeze the bottle, water is forced into the pen cap. This increases the weight of the sub and it sinks!