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Believe it or not, plain old tonic water glows under a black light — quite brightly, as well. To get this effect, start by pouring some tonic water into a container where you can see it. You can add it by itself or dilute it with water. However, the more water you add, the dimmer it will glow. Tonic water is available at most local stores and supermarkets for just a few dollars. Be sure to get tonic water, not club soda or soda water. The bottle should say "with quinine" or something similar. All you need to do to get tonic water to glow bright is to illuminate it with a black light. Be sure to dim the lights in the room before you do this or it will be more difficult to see the glowing effect. Black lights are available from specialty party stores (like Spencer's, etc.) or online. The price for the black light often depends on its size and brightness — basic lights can cost as little as $20 or less. Making tonic water glow with a black light makes it look very strange, but it doesn't make it poisonous, radioactive, or harmful to drink in any other way. However, tonic water is often high in calories and sugar, so enjoy it sparingly. Tonic water glows this way because of tiny chemicals called "phosphors" inside the liquid. When ultraviolet light from the black light (which humans can't see) hits the phosphors, they convert it into a form that humans can see, producing the glow.
Pour tonic water into a clear container. Shine a black light on the tonic water. Don't worry about drinking the tonic water.