Our measurement partners have all sorts of clever ways of accurately working out audience numbers (or penetration, as they're rather fond of saying).

Some of these are a bit secret. If you really press us we'll tell you, but the general ones include:

• Tracking websites - seeing what websites a viral has been viewed through, and then analysing where in the world and what sort of people and age groups have watched it.
• YouTube views - from our knowledge of YouTube demographics, we can work out statistics for specific audiences
• Viewer ratings - YouTube allows viewers to rate every film out of 5. For example, Steamy windows was rated an average 4 out of 5 by all of the 1300 plus viewers who rated it.
• Feedback - virals give you a unique opportunity to get user comments. How many other media do that? And people really do feel the need to comment.