Pilot Frixion 0.5 mm

Erasable pen from Japanese stationery brand Pilot.

Heat sensitive ink that disappears when heat is applied using the eraser on the back of the pen. 

Ink disappears when temperature reaches 60°C and reappears when temperature go down to -10°C. Erased something important or left the writing inside of a car on a hot day? Pop it in the freezer for about an hour to make it appear again. :)

0.5 mm is considered ultra fine in US standards. Regular medium ball point pen is usually about 1.0 mm and fine point is about 0.7 mm.