People strolling the streets of Utrecht and Leiden recently came across entirely red or yellow mobile posters, with no other image or logo in sight. They were located a short distance from McDonald’s restaurants, to which people who’d passed the posters now felt irresistibly attracted. It worked like this: the posters emitted the odour of burgers and fries. As researchers have long understood that smell is a trigger of appetite, the wafting odour of a snack, combined with the familiar McDonald’s colours, sent those who’d been exposed scurrying to the chain to satisfy their craving.