I've been a teacher for a year, and I've taught about 1200 different people since then. Imagine seeing one of your ex-students in a hospital, shopping center, store, etc. Imagine that you remember the face but not necessarily the moment you met that person. I cannot say hi, even though I know him/her because it would be extremely weird. Consider I'm 19 and I'm usually on casual clothes, and their age range go from 9 to 70.
