Another missed joke

[davewitley]

Can't tell if this is on-topic or off, but I reckon any replies will 
almost certainly be off-topic, so here it goes.

Question for US (well all non-British) members: do you use the 
expression 'take the piss' to mean to mock, or wind up, usually good-
naturedly?

It's another of those HP jokes I hadn't quite grasped until now.  
When Dumbledore tells Karkaroff about the chamber pot room, Harry 
thinks he is joking, to Percy's disapproval.  What I only just 
noticed for the first time is that Dumbledore's comment about a full 
bladder is a signal that it *is* a joke.  He is saying, for those 
with ears to hear, that he is taking the piss.

David, who thinks Jessica and Jo have conversations like:

'Mummy, I bet you can't make Harry do a poo in the next book!'
'I bet I can... Now, let me think...'