I am working on a project in which I must move an object a moderate distance (left to personal preference) using buoyancy as the main mechanism of movement. I have a few ideas for how to accomplish this, but I am having major problems thinking about how to implement aspects of each. Below I describe three of these ideas. Any input as to which idea is most feasible, and how to resolve the major difficulties, would be appreciated. Every idea involves an open, narrow tank of water (like a small aquarium) in which an object enters at one side of the tank at the surface, moves beneath the surface, and then resurfaces near the other side of the tank. Idea 1) An object falls into one side of the tank and sinks due to its density being slightly more than that of water. A propeller located underneath the water near the bottom creates a current parallel to the bottom that the object travels in to move across the tank under the water. The object's volume is increasing during this time, so that, although its mass is unchanged, its volume has increased and its density has decreased, and now it is a density less than that of water, so it floats back to the surface. My major problem with this idea is that I cannot think of any object that expands in water without its mass changing. If there is any object whose density decreases after being submerged in water, or whose density can be made less, then I can use this idea. Idea 2) A ping pong ball rolls into one side of the tank and floats. A flat wooden plank is dropped into the tank with its length parallel to the surface of the water. The plank falls straight down and sinks, pushing the ball underneath it. The wood is a density that allows it to sink deep beneath the surface but not fall the entire distance to the bottom. Underneath where the wood stops, there is a propeller creating a current. The ping pong ball rides the current out from underneath the span of wood and then floats to the surface. The major problem with this idea is finding wood, or some other material the right density, and figuring out how to make the ball move without substantially affecting the wood. Idea 3) An object slightly more dense than water enters the tank on one side and sinks. As it sinks, it rides a current produced by a propellor across the tank. At the same time, salt is being mixed into the tank to make the water more dense. At a certain point, the object is less dense than the salt water and rises. My major problem is making the water adequately dense in a short amount of time. The entire process of density change can take no longer than 10 seconds. Perhaps there is a different substance that can be mixed in the water that takes less time to mix than salt. Overall, my goal is to demonstrate buoyancy while moving an object. Ideas using propellers, tanks, and circular objects represent the best I've been able to come up with, but I am open to better ideas of any kind.