Last year my friends and I formed a club for (as Cory puts it) "people who aren't good at magic tricks." (Actually, Cory, John Edgar Park, and I are the only ones in the group who aren't good at magic tricks. The others are pretty accomplished magicians and passed the audition to become members of the Magic Castle.)
At our last meeting Michael Borys introduced me to the FriXion pen. It's an erasable pen made by Pilot. It comes with a small eraser, but you can buy a large eraser, which is a smooth brick of plastic. When you rub a mark made with the pen, the friction creates heat to erase the mark. The cool thing (or bug, depending on your use case) is that the writing will vanish instantly when you apply heat. It's a heat-activated disappearing ink. I read that if you apply ice to the erased writing, the writing will reappear (it will be faded, however).
Amazon sells a 3-pack of the FrXion pen for $4, and a 4-pack of erasers for $6.
