Because apparently, some people our age still do not know about the Pilot Frixion Pen ((At the time of writing, this ebay shop is Germany's cheapest option.)), let me just very briefly inform you about this ingenious item. It is a regular ball pen, but the ink it uses turns invisible under a certain amount of heat. Hence, the Frixion has a little rubber nob at the end, which effectively works as an eraser, because rubbing it against a sheet of paper produces enough heat to turn the ink invisible! Good news: Leaving your lecture notes on the heating does not erase them. Seriously though, this pen is just awesome.