Doodling with UV Light Reactive Ink with a glass pen like this makes every drawing a little more magical. You can also use it with fountain pens. But in order to work, you would need UV Black Lights.

UV Light Reactive inks are clear and appear invisible under normal light. When exposed to UV black lights, the react by changing into vivid colours.  When the black light falls upon the UV light Reactive ink, it makes the ink fluoresce. Practical uses for these are in paintings or secret messages which can only be seen under a UV light.