Pen Uni ball Signo Erasable Gel Ink Rollerball

The Uni-Ball Signo TSI (Thermo-Sensitive Ink) uses a specially-formulated ink, which changes from coloured to clear when heat is applied. Simply by rubbing the ink line with the cap of the pen, enough heat is generated to turn the ink clear - 'erasing' the line cleanly. The same pen can then rewrite over the erased lines. Erased TSI lines will reappear if subjected to very low (-18 °C) temperatures.
When erasing, the ink initially disappears much quicker than other erasable pens.  Don't be fooled though - a little extra heat is still needed at this point to stop the ink colour from coming back: just give the writing a couple of extra rubs and it'll be invisible for good (unless you put it in the freezer of course).

There's an eraser on top of the cap and one on the end of the pen, handy for those that like to place the cap on the end of the pen when writing as you'll not have to remove the cap every time you need to erase something!