The next time you go back, some of the cleverest software in existence remembers you, and every time you click on a video, complex algorithms offer you a personal menu of what might get your rocks off, based on other users' data - only this time the heat is notched up. By continually offering racier porn, so begins the path towards converting you into a paying customer. An estimated 10 per cent of porn users will ultimately pay. Maybe not this week, or even this year. Maybe not ever. But the porn providers, who are in it for the long haul, are patient.