I am his tutor and he is trying to tell me about an unknown variable. About X. But he has forgotten that it’s called X.  “The mysterious thing,” he says, laughing.  I love him for this. I will tell everyone I know about the mysterious thing.  During one session we’re in his apartment and I hear a beep. Just one beep. The microwave, probably.  “I’m really sorry,” he tells me, tensing up.   Sorry for what? It feels like I’m missing something.  “Totally fine!”  On the walk home I wonder why he was so on edge. Then I forget about it,…