I did this by accident when I was younger. I was playing with my blacklight and some highlighters (writing out secret messages and what not) and the marker I was using was starting to dry up. I usually just wet my fingers and use them to wet the tip, but someone had left a small glass of water on the table so I instead dipped it into there (kind of reminds you of a feather and inkwell). After a while the marker would start to go dry again, so dipped it again....and again. I decided to let the marker soak up some of the water in hopes that it would last a little longer so I placed it in the glass and switched over to another colored marker. After about 5 or so minutes, I looked up and saw the water "glowing" a little. I placed my light closer and it started to really glow. That's when I discovered that highlighters make really cool effects when used in water.
Here's what you'll need:
1. Highlighter marker
2. Glass of warm water
Before I forget to add this in.... Warning: Don't drink the water unless you used non-toxic highlighters (even then it may not be a wise idea to do)