Dancing Raisins.

Equipment: Jar of water, a jar of clear fizzy drink, a handful of raisins. The raisins are heavier than the drink so they sink to the bottom. At the bottom of the fizzy drink they collect bubbles of carbon dioxide and now the 'raisins and bubbles' are lighter than the drink so they rise to the surface. When they reach the surface, the gas bubbles burst and the raisins sink, then they collect more gas bubbles etc. This will continue as long as the drink is quite fizzy.