What's the world's most profitable product?

BBC's Rory Cellan Jones reported that Apple's profits had fallen for the first time since 2003 and speculated that the world's most profitable product was the iPhone. Apple profits from the iPhone are thought to be $44 billion - so does that make the iPhone the world's most profitable product? Or is it Coca Cola, Microsoft software, Pfizer's Viagara - or even lemon cake? Laura Gray and Tim Harford investigate.