I met him because he got his tones wrong.
On Tantan, his profile said: Historian. Leipzig, Berlin, Shanghai. Looking to practise my Chinese, correct me without mercy. A photo in front of a library, another on a bicycle, a third where he was reading in a café and pretending not to see the lens. The boys at Fudan post photos of cars. He posted books. I was twenty-two and I found that elegant.
I corrected him without mercy. He mixed up 买 and 卖, buy and sell, which, in hindsight, should have served as a warning. He answered in German, as a joke, and I spent the evening with a translator working out that he'd written: "You are stricter than my professors." He'd used the formal you. Nobody uses the formal you on Tantan.