Why do raisins bob up and down in carbonated water? Yesterday I preformed a fun little experiment where I poured a glass of seltzer water into a glass, then dropped a few raisins into it.

I observed that the raisins would float at the top of the glass for a few seconds, then sink all the way to the bottom, then float back up, and repeat seemingly without end.

Why does this happen?
The raisin has nucleation sites on it that allow bubbles of $\ce{CO2}$ to form. The raisin is light enough to be lifted by the bubbles as they push their way to the surface. As the bubbles are released into the atmosphere, the raisin once again sinks until more bubbles form on it. This will continue until the soda water has lost the majority of its dissolved $\ce{CO2}$.