Today, given the folkloristic recurrence, I wanted to see if my papirology professor (whom I've never even seen smiling) had any trace of irony in his soul.
Thus, during the five minutes pause in which he goes outside class, I put a small (1.5 inch long) paper fish figurine quoting "IXTHUS" (fish in greek) on his desk.
I rather believe this'd been a delicate and polite sort of joke (I don't believe this is worth being described as a joke, more of a divertissement) but it didn't got the reaction (or lack thereof) I expected.
"Humpf, like kindergarten kids"
Geee... I wonder if that person even knows the meaning of "smile".
This even however has shown me another useful use of my being an invisibile, as I put the fish on the desk in front of everyone without even trying to hide, but at the end of the lesson no one seemed to know who did what.

I have a career as a ninja :D