Now you see it, now you don't! Erasable ink pens have come a long way since they first appeared in the late 1970s. Back then, erasing errors seemed to be more a case of rubbing briskly and removing strategic paper fibres - thus removing the error along with them - rather than actually erasing the ink. These days, friction is still used to remove the ink but in a more aesthetic and far less messy manner. The key? Thermo-sensitive ink. Applying friction to anything written in this specially-formulated ink - usually by using the pen's incorporated eraser - results in a rise in the temperature of the ink, which consequently becomes translucent. Lines are therefore removed quickly and cleanly, and what's more, if you stick your paper in the freezer, the drop in temperature means the lines will magically reappear!