Look who’s behind you.

I was studying with my friends at school. We were talking and laughing about, when a teacher of one of my friends suddenly appeared and started talking to us. But he was very crazy and all he said was pure nonsense. We thought he was trying to crack a joke but it just did not work. Poor him, all he could do was to laugh by himself, so all of us also laughed just to help him.
When I thought he was far enough, I said :
‘He’s so strange!’
When I turned back, who was there right behind me? The teacher himself! O:

( Always look who’s behind you before you start talking about someone )