Professor (to a student): Okay, now uncap the nitrogen flask and fill the canister.
TA: Actually, I think there's already nitrogen in the canister, Professor.
[Professor grabs the canister and quickly dumps its contents on the floor - it flows out towards the students, then disappears - hey, it's nitrogen, right?]
Professor: No, there isn't.
TA: Right as always, sir.

(earlier)

Professor: So if current is like water flowing in a pipe, what is our electrical analogy to pressure? You. [Points at a student]
Student: Uh ... Coulombic interactions? Electric field? The conductor?
Professor: Are you going to rattle off all the nouns you've heard this semester, or should I just tell you the answer?

(This substitute professor was actually a neat dude - way better than the usual lecturer.)