In marketing and advertising, it is all about going viral on the internet, getting your audience to do your distribution for you. Is there an art to it or is it down to plain luck? The BBC's Sarah Stolarz has a special report. Plus, we speak to the journalist and media consultant Martin Belam, and Will Hayward of Buzzfeed, about sites specialised in getting as many people as possible to share their content. How do they work? And how are they changing the businesses of news, marketing and advertising?