There's a pen, made by Pilot, where the ink is "erasable". You can buy it here at Jet Pens
The funny thing is that the ink is actually erased by the heat of friction, and it reappears
if you cool the paper down below 14° F (-10° C), which is pretty easily done in most freezers (or outside today... whew it's cold). So it kind of works as a disappearing ink that's the opposite of the usual kind of disappearing ink (most of which appear with heat or UV rays or... ).
Speaking of the Cold. It's been -10° F (-23° C) outside at night or in the early morning. Jet and I have been blowing bubbles outside, catching them on the blowing ring, and watching them freeze. It takes a little while, and you can also blow them onto a pan of water (so long as the water doesn't solidify on you). You can see the crystallization of the soap solution and then the whole bubble shrinks just a little before the film goes solid. Usually they just collapse, but occasionally you'll get a really strong bubble in which holes will appear and just stay and the rest of the structure remains stable. Jet's even poked his finger through the ring of the blower and held it there with the rest of the bubble all around it. It's really... cool. (or so she says as she's freezing her BUTT OFF... ahem)
Hope you're all doing well! Hope Thanksgiving was fun and that the shortening days are inspiring you instead of bringing you down. I managed to do Nanowrimo before I left for San Diego, but I'm sitting here with half a novel of my David finally finding his true love (and Afghanistani fissionables, Ukrainian nuclear power, and Russian politicians who were once Russian Bratva) and wondering if it's even what I really want to do to finish it, but I might as well just finish it so that it's done and David is finally settled down with the love of his life, and then get on with the 'real' one. *laughs* I may just finish it so that I can publish it as a weekly somewhere (including here, since I'll likely do it for free). Nanowrimo certainly made me realize that there's lots of ways to publish on-line content.