How do erasable pens work? I understand the principle behind an eraseable pencil. Graphite is highly slippery, you rub it onto the paper, and an eraser can rub it off again. 

I've had friends at school with 'eraseable ink pens'. The ink was 'special' and the eraser was 'special' and it could erase the pen in the same way a regular eraser could erase pencil. 

My question is - how do eraseable pens work? (Ie what substances are involved and what are their properties?)
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.