Put on latex gloves.

Remove a fluorescent highlighter pen's interior plastic tube that is filled with fluorescent-dyed cotton. Pop off the pen's bottom to access the tube.

Catch water in a clear glass jar from a slowly dripping faucet.

Hold the plastic tube from the pen underneath the faucet's dripping water so the water seeps through the fluorescent-dyed cotton and into the glass jar.

Squeeze and knead the plastic tube until all of the dye releases from the cotton into the jar.

Remove the tube's plastic casing to expose the cotton, allowing the water to rinse away all the remaining dye.

Hold the jar containing fluorescent-dyed water in front of a light-up LED key ring or other item with a blue LED light source as you squeeze its blue LED light. The water will glow. A light-up "ice cube" or simple nightlight with a blue LED light are options to using a light-up LED key ring. The blue light from a projector works very well as the light source for this project.