A jar of clear fizzy drink, e.g. 7-UP or soda water
(freshly-opened: must be very fizzy)
A handful of raisins
The raisins are heavier than the drink so they sink to the bottom. There they collect bubbles of carbon dioxide and now the â€˜raisins + bubblesâ€™ 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.
Drop a handful of raisins into a jar of freshly opened fizzy drink. After a while the raisins will rise to the surface, sink, rise again, etc. thus dancing up and down.