Chemistry - How do erasable pens work?

Solution 1:

Instead of a typical oil/dye ink, erasable pens use a compound closer to rubber cement, which acts like a liquid when a bit of pressure is exerted. 1

A bit of history and a link to patents can be found here

EDIT: Be sure to check out the other answer here about thermochromic ink too.

Solution 2:

There are two types of erasable pens: rubber cement and thermochromic pens. Thermochromic pens are much more popular nowadays because rubber cement pens literally harden like cement, and can't be erased after a few hours. Thermochromic pens, however, use heat from friction that makes the ink invisible; not exactly removing it from the paper, but close enough.

One downside to thermochromic ink is that the ink can fade over time and rubbing the paper too much can make it invisible, but you can undo this by placing it somewhere extremely cold (think about $\pu{−10 °C}$ (freezer)) and it should reappear.