Invisible Ink

Have you thought it would be cool to write secret messages in invisible ink a lá Ford Pines in Gravity Falls? As it turns out, making UV-reactive invisible ink is actually pretty easy.

You can buy reasonably inexpensive black light pens on Amazon. (Starting at around $6 per pack of ten.) If you live in the US, you might find them in the toy section at Walmart. 

But as for the ink itself, all you need is a bottle of diet tonic water. Simply boil it down until there’s only a few tablespoons left. It should still have the consistency of water, but with a faint yellow tinge. (Note that using a wide pan will help the water to evaporate faster.)

You can then write with it using a paintbrush, or pour it into a fillable paint pen. When the ink dries, it will invisible until you shine a black light on it.

Here’s the science behind it: the fluorescent ingredient in tonic water is quinine, a bitter-tasting compound that comes from the bark of the cinchona tree. Since it’s edible, you can even use this ink to decorate food products such as cake fondant. (A cake with glowing lettering will look magical at a black light party! You can thank me when you're a hit with the kids.)

Et voilà: you now have a new way to make notes in book margins, highlight passages and write secret messages– both on paper and pastry!