In a recent UK poll, the number 7 topped the list of people’s favourite or lucky number. But why? What is a mid-life crisis, and does it really exist? Researchers have tracked the happiness levels of thousands of people and confirmed the age-old suspicion of a dip in human happiness during middle age. In Twitter Time this week, the truth behind healing benefits of purring cats, plus an overview of hippy dribble practices, and how the placebo effect works.