Why do some questions not start with an auxiliary verb?
When I learned English, my teachers told me that all questions **must** have an auxiliary verb at the beginning, just like *Are you mad?* or *Is she playing?* do. But when watching some movies or talking with people who speak English, they just ask using things like *You mad?* and *She’s playing?* Of course, the kind of intonation let me know they are asking a question, but why this happen?