Sharpie markers are well-known for being indelible, particularly on plastic, glass and metal surfaces. Folks in labs, movie sets, and hospitals who need to mark things permanently use Sharpies. If the ink goes on, it won’t come off. What’s special here is that the other tip of these pens is an ultra- fine point Sharpie, fine enough to write like a ball-point pen – but permanently — when you need to. The “industrial” version of Sharpie ink will even resist chemicals and scrubbing. Since more writing surfaces seem to be plastic-like, I find we use Sharpies all the time now.