cubes, and black light to a dark room.
drop some tonic water ice cubes into one of the jars of water, and drop some tap water ice cubes into the other jar of water. observe both under normal lightening. observation #1: ask the kids if the ice cubes are floating or sinking.
now turn off the lights and shine the black light on the jars. observation #2: you should see the tonic water ice cubes glowing but the tap water ones are not.
shine the black light on the ice cubes still left in the trays. only the tonic water ice cubes will be glowing.
watch the cubes melt as long as you like. 