Twitter, the 140-character chat site where celebrities can talk to their fans, has become a format that is improving student learning, a U.S. researcher says. College students who tweet on the social network as part of their class instruction are more engaged with the course content and with the teacher and other students and achieve higher grades, Michigan State University education Professor Christine Greenhow reported. \"Tweeting can be thought of as a new literary practice,\" Greenhow said. \"It\'s changing the way we experience what we read and what we write.\" Twitter use among U.S. teens has doubled in less than two years, she said, with more than 200 million active users now posting more than 175 million tweets a day. Twitter\'s real-time design allows students and instructors to engage in sharing, collaboration, brainstorming and creation of a project, Greenhow said, and helps them learn to write concisely. Greenhow said her students in a class focusing on Twitter participated more through the site than they did in a face-to-face class setting. \"The students get more engaged because they feel it is connected to something real, that it\'s not just learning for the sake of learning,\" Greenhow said. \"It feels authentic to them.\" \"One of the ways we judge whether something is a new literary form or a new form of communication is whether it makes new social acts possible that weren\'t possible before. \"Has Twitter changed social practices and the way we communicate? I would say it has,\" she said.