what would you do in this situation... besides laugh?

Okay, so here's the setup:

 

  Monday, my first day, I meet this one student... we'll call her Li'l-Miss-Wacked-Out (LMWO).  She comes in late for class and says that she already has the assignment done, but it was for another class, and is it alright to just use that one.  I say sure, just make sure it's relevant to this assignment.  She assures me it is and proceeds to sleep in class for some time.  Every now and again I'll accidentally "trip" over her chair to wake her, or yell "LMWO, you're drooling on the keyboard!" to keep her conscious.  It sorta works.

 

  Tuesday, I don't see her.

 

  Today (wednesday), part way through the class she looks at me with a total disbelief look and... well I can't do LMWO word-for-word, but here's the gist:

"Who are you?  I don't remember you!  I apparently came in late to this class 2 days ago, but I don't remember.  Apparently I was drunk or something, from what people are telling me.  But I did find this sheet in my binder [it's a sheet describing the assignment she was to be doing], what is it?  What does it mean?  I don't remember anything!"

 

So... how do you keep from laughing?  I just nodded now and again through the ~5 min explanation/mind recovery exercise and when I had the opportunity I deked out and had to answer another question.  Man... I nearly lost it while she's trying to explain.  The only problem was that she had the total look of seriousness... the look as if she REALLY didn't remember anything, and was hoping she hadn't killed anybody.  Pretty freaky...

 

Who said teaching can become blase?