Harry Potter joke

[Jeralyn]

So.

I'm currently rereading GOF, and have stumbled upon something I think would 
be fun.  In the opening feast, Dumbledore begins a joke:

"...I did hear an excellent one over the summer about a troll, a hag, and a 
leprechaun who all go into a bar..."

Unfortunately, McGonagall cuts him off before he can finish.

So your mission, should you choose to accept, is to finish the joke.  Any 
takers?