Social media has changed the world. The USA's top doctor said it can cause mental health problems in young people. Dr Vivek Murthy wants mental health warnings on social media sites. He said youngsters see too much sex and violence online. He wants warnings that say social media can harm mental health.

In Dr Murthy's recent warning, he said: "The mental health crisis among young people is an emergency." He said young people spend too much time online. He said teenagers "who spend more than three hours a day on social media [could] double the risk of anxiety and depression".