In this experiment you can observe bread balls going up and down in water. The carbonated water is an carbon dioxide oversaturated solution. An oversaturated solution isn't stable because the solute (gas) will always have a tendency to abandon the solution. Therefore, if you leave the carbonated water open to atmosphere, after some hours it ends losing all the contained gas. The loss of gas can be accelerated with the addition of extra nucleation points.
When you add bread balls to carbonated water there will be extra nucleation points and it will cause the formation of small gas bubbles in its surface. Then, the bread balls and gas bubbles group will have lower density than water. Thus, the group arises to the water surface due to the density difference. In the moment that bread ball arrives to the surface, the bubbles of gas are freed to air. After that, the bread begins to dive again because its density is higher than water. After some time of down movement, it begins the formation of new gas bubbles in the surface of the bread, which cause the reverse movement again (up).
Apparently, this process seems to be cyclic because it always returns to the initial point. However, this cycle is irreversible because with time it is less and less intense. During the experiment, the water will have less gas to free after each up and down movement.