The key to erasable pens is that they don’t use real ink. Normally, ink is made out of dyes and oils that leave a mark as the pen moves across the paper. Ink will smear or smudge if rubbed with an eraser but will not come off the paper once applied.
Writing With Glue
Erasable pens are filled with a liquid rubber cement. Color is added so that it looks like ink when the pen is applied to paper. When erased, the rubber cement is rubbed off the paper. This only lasts until the rubber cement hardens, which can take up to 10 hours. After that point, erasing won’t work.
Hot New Technology
A newer version of erasable ink -- "thermochromic ink" -- relies on the eraser creating heat when rubbed against paper. When the ink is cool, it is dark and shows up on paper. Heating the ink turns it clear. The heat from an eraser is enough to make the ink seem to disappear.