not a magic trick but very impressive all the same (we did it for a science demo. at daughter's school a couple of weeks ago).
take a 2 liter bottle of diet soda (needs to be diet, any type works). put bottle on flat surface, open lid, drop in 2-3 mentos (those mint candies)-STAND BACK!!! the soda shoots upward like a roman candle. this empties about a full liter of the soda (upwards) in a matter of seconds.