What you'll need:
A black light (you can find them at places like Walmart and hardware stores, as well as online stores like Amazon).
Tonic water or a highlighter pen.
A dark room to do the experiment.
If you are using a highlighter pen carefully break it open, remove the felt and soak it in a small amount of water for a few minutes.
Find a dark room.
Turn on the black light near your water, how does it look?
If you us a highlighter pen use a knife to (carefully) cut it in half. Remove the felt and soak it in a small amount of water for a few minutes.
Once you have the dye you can add it to more water to make glowing fountains, grow certain types of glowing crystals, make glowing bubbles, and use it for many other water-based projects!
Make Glowing Water