A billion people across the world smoke cigarettes, and many would agree it’s the hardest habit to quit. One such smoker, listener Sharif, emailed CrowdScience from Uzbekistan to ask if we could find out why giving up is so difficult. Marnie Chesterton travels to San Francisco to meet addiction experts and discovers how nicotine tricks smokers into thinking tobacco’s good for them. And we meet ex-smokers at a weekly therapy session aimed at retraining the brain. Do you have a question we can turn into a programme? Email us at crowdscience@bbc.co.uk Presenter: Marnie Chesterton Producer: Marijke Peters (Image: Lit cigarette. Credit: Getty Images)