Tonic water contains quinine,
a fluorescent alkaloid. Put some in the path of a UV light, and you'll see a beautiful bright blue beam. You can create fantastic effects in a gin & tonic with a UV keychain flashlight - serve some up at your next party, or just take along the flashlight next time you go out, and order a G&T (you'll soon find out if you've been served a gin & soda instead - it won't work!). Note: you'll see this effect any time you have a tonic-containing drink under a blacklight. See How to Make Edible Glowing UV Reactive JELLO, for example.