Do you know when an advert is really an advert? Can you be sure that the game you're playing isn't trying to make you buy something? When it comes to protecting our children from sugary food, the world of online advertising is the new frontier. Harry Wallop investigates and finds big name brands marketing fattening food in the games children play. Dispatches goes undercover in the ad world, creating a high-sugar drink to see who's willing to promote it to young children, and revealing the tricks of the trade.