Found a great pen to mark HP calculator cards

[Xorand]

(07-27-2022 07:31 PM)ijabbott Wrote:  The other nice thing about these pens is they use a bistable invisible ink. Heating the ink up by friction (using the "eraser" at the end of the pen) turns it transparent. Cooling it below -10C for a while turns it visible again!

Interesting. I knew about the friction part "erasing" the ink. Didn't realize it would come back by freezing it.