For the purposes of this joke:
carafe \k*-'raf\, n. [F.] A glass water bottle for the table or toilet
Some medical students once noticed that their lecturer was in the habit of always taking a sip from his glass of water midway through the lecture.
[Dr Tapp takes a wry sip from his glass of water]
Being medical students they decided to make use of this in order to play a trick on him.
[Thus far, people are uninterested in the words of this much acclaimed comic genius]
One morning, a group of them decided to sneak into the lecture theatre early. They crept towards his carafe of water.
[The word carafe has incited much confusion and consternation amongst the students]
And do you know what they did?
[Here we go again...]
Well, do you?
[Dr Tapp's legendary rubber jowls are exploding outwards in bursts of jollity]
I'll tell you what they did. They replaced the contents of the carafe with vodka!
[Dr Tapp takes another sip of his drink, still further prolonging our agony]
Their lecture proceeded with its usual alacrity, until the much-anticipated break arrived.
[We're on the edge of our seats. We really are. Several Clare students are laughing as they remember Dr Tapp drunk at the previous evening's party]
The lecturer proceeded to his carafe as usual, and poured out a glass.
[Dr Tapp takes what seems like an eternity in pouring himself yet another humourous glass of water]
He raised it to his lips, and took his customary sip. And do you know what happened then?
[By now we're used to his rhetoric style and inimitable comic timing, so we just sit and wait for the inevitable]
Well do you?
[A small procession of students enters the room as those who left at the start of the joke wander back in with sandwiches from Nadia's]
Well I'll tell you. That's when they realised...
[A hushed silence develops over the lecture theatre. Several students have now fallen off the edge of their seats]
...it had always contained vodka!
Carafe - may or may not contain vodka