Lighting a cigarette releases thousands of different molecules, so it might seem unlikely that any single molecule would be a unique marker for tobacco smoke, as opposed to smoke of any other kind. Surprisingly, Juan M Sanchez and colleagues of the University of Girona, Spain, found from samples of air in 56 local cafes and restaurants that the simple molecule 2,5-dimethylfuran can give the game away. This molecule seems to be unique to tobacco smoke and is not found in smoke from cars, cooking or other types of combustion. Looking for this substance is even sensitive enough to detect smoke in the breath of non-smokers who work in smoky environments.