These inks are supposed to be invisible unless activated by light or heat, but in practice, you can usually still see a little bit of what’s written with the ink.

Geek Life: Fun stories, memes, humor and other random items at the intersection of tech, science, business and culture. SEE MORE Lines drawn with Living Ink’s algae pen disappear in about 10 ...

In this challenge, I used a Smiggle Spy Pen to draw a full doodle using invisible ink that only appears under UV light! 🔦🖊️ Drawing without seeing the full lines in real time was tricky—but the ...