How do you make a submarine that floats, sinks, then floats again?

Chemical reactions have to be contained. Nothing can enter the water. No dissolving materials are allowed. It has to float for at least 3 seconds. It has to float, sink, and float again in less than 5 minutes. The sub has to be self-operational. It cannot be touched once in the water. Nothing can be dropped or taken out of the water. The sub cannot cling to walls or be connected to anything outside the tank. The tank is 45 cm deep. The density of the water must remain unchanged after the sub is done. Please explain in simple terms that are UNDERSTANDABLE. No gibberish. Explain materials and procedure. Explain how the sub works.