Poke four holes along one side of the water bottle. This will be the bottom of your submarine.
Pour a tablespoon of baking powder into the bottle so that it settles over the holes.
Add five marbles to the bottle. This will help add weight to the bottle and keep it from rolling over on top of the water. Put the cap on and tighten it.
Place the bottle bottom-side down into a bathtub filled with water. Water will fill the bottle through the holes in the bottom, causing it to sink. When the baking powder reacts with the water inside the bottle, it will release carbon dioxide gas. This will create bubbles and cause the bottle to rise back to the surface of the water. This process can happen several times before the baking powder is completely dissolved.
Change the amount of baking powder and the number of marbles in the bottle, and record the changes. Keep track of how the changes affect the submarine, such as how many times it resurfaces or how long it takes it to resurface under the new conditions.