Here's a rather long joke...

Two men walk into a pub, and all the inhabitants greet the one called George, and the other asks George:
"How do all these people know you?"
George replies "Everybody knows me."
the other says "You can't know everybody."
And George challenges him "Name any person that you think I won't know, and I'll prove I know them."
And so, the other guy accepts, and says the Prime Minister.
They both go off to London, and George goes up to the police guarding, who says "Is that you George? Long time no see, mate. You off to see the PM?"
and George says "Aye, and this is my mate who doesn't believe I know everyone, so I'm proving to him that I do."
So the pair sit in No. 10, and have a cup of tea with the PM.

The next day in the pub, the other guy says "Well, you know the PM, but I bet you don't know the Queen"
And George says "What? Me and Lizzie go way back"
So the pair set off to London once again, and go to the Palace, and George gos up to one of the Scots Guardsmen at the gate who says "Evening George. You off to see the Queen today?"
and George replies "Aye, that I am. And this guy doesn't believe that I know everyone, so I'm proving it to him."
So the pair walk into the palace, and go into to State room, and the Queen is sitting there, and she says "George! I have not seen you for a while. Come and have some tea."

The next day in the pub, the other guy says "Well, there is one person you cannot know: the Pope"
To which George replies "Weesht, 'course I know the Pope. I was there when Benedict became a priest, nevermind when they Poped him."
So the pair set off to Rome, and George goes up to the Swiss Guardsman in the Vatican, who says "George? You wanting to see His Holiness? He's up on the balcony (the one at which the Pope often appears) just now, just go up and meet him."
and George says "Right, and my mate's just gonna come with me--"
and the Guardsman interrupts "Sorry mate, but he can't go up"
So George turns to the other guy and says "Right, you just wait down in the crowd and watch while I talk to the Pope"

The Other Guy is in the crowd, and he sees George up on the balcony talking away to the Pope, but for all he knows, the Pope could be telling him to bugger off. So, the Other Guy turns to a random guy next to him and says "Do you know that guy up there?"
and the random guy next to him says "What? That guy talking to George?"