What is multiple sclerosis (MS), what causes it, why do some people suffer from it, and how can we treat it? This week we hear about a drug that can halt the disease in its tracks for some patients, and how scientists screening chemicals that trigger the brain to make new myelin have stumbled on a therapy that might reverse the symptoms for some sufferers. Plus, in the news, why the US and India are launching probes to Mars within weeks of each other, where the meteor which exploded over Russia in February 2013 came from, how Prozac makes the brain more plastic, and the stem cells which mothers give their babies in breast milk