1) Drill a small hole in the lid of the film canister using a drill
2) Add just enough pennies to the canister so the it will sink. Remove one of the pennies, so the canister will now float.
3) Fill the canister with water, drop in a small in a small piece of the Alka-Seltzer tablet and then replace the lid
4) Quickly drop the canister, lid side down into the cylinder/tank that is full of water. The canister should sink to the bottom, and in a short time it will rise back up to the top.
5) Try other fuel sources such as Baking soda, Citric acid or Baking Powder
When the water and Alka-Seltzer are added, CO2 gas will be produced, which will force water out of the small hole. It is essential that the hole be pointed downward, so that the water can be pushed out by the gas. If the hole is point upward, the gas will simply bubble up through the water without displacing it.
When the water is pushed out of the canister, the canister mass is decreased. Since the volume of the sub remains constant, the density of the sub is therefore decreased.
NOw that the density has decreased, the buoyant force of the water is able to push the sub upward, causing it to rise. This is how real submarines operate. To sink, they take water into their ballast tanks, making the sub less dense than water.