The old color changing milk trick amped up with Florescent Inks from highlighters! Toss in a blacklight (OK.. not literally, but hold it above) and REALLY see a show! This is a great experiment to show your kids... or for the kid in you. Groovy man!
KEY TO THE TRICK: Heat the milk! The hotter, the faster the reaction when you add the drop or two of dishwashing liquid!