Heat up 1 cup of tonic water over the stove until it begins to boil.
Pour the flavored gelatin packet into the glass container. Carefully add in the boiling tonic water and stir until all the gelatin is fully dissolved.
To help cool the mixture, slowly add a second cup of (non-boiling) tonic water. Be careful when pouring since the solution might fizz.
Tonic water will make your gelatin glow more brightly under a black light, however, it will have a slightly bitter taste. You can replace this cup of tonic water with normal water if you prefer.
Once the entire solution is mixed, place the gelatin mixture into the refrigerator and let it rest for about 4 hours or until firm Then, take out your solid gelatin and place it under a blacklight. You should see it glow!
What is going on?
Why does this gelatin glow? The active ingredient in tonic water is quinine, a fluorescent substance. This means that it becomes excited as it absorbs energy from other light sources and then releases light as it returns back down to its calmer, natural state. In this case, the quinine absorbs the energy from the UV light and emits some of that energy back in the form of visible blue light. This is what gives your gelatin its glow!