Some teachers, I know them and they know me too, for example, my teachers and classmates in XJTU, OGI, and BiMBA;
Some teachers, I know them but they don’t know me, for example, my teachers such as Zhongtian Yi in Lecture Room or Lynda.com;
Some teachers, I never know their names, for example, roles in movies or TV shows I took a look, people in parks or on streets, etc.;
Some teachers can be carried all the time, such as books, iPad, smartphones, etc.