Why is a joke?

In Obama's Westminster Hall speech, he began with:

"My Lord Chancellor, Mr. Speaker, Mr. Prime Minister, my lords, and members of the House of Commons:

I have known few greater honors than the opportunity to address the Mother of Parliaments at Westminster Hall. I am told that the last three speakers here have been the Pope, Her Majesty the Queen, and Nelson Mandela -- which is either a very high bar or the beginning of a very funny joke."

Everyone laughed here. I don't know what is funny here. Could anyone help to translate it

into Chinese or explain ?

Thanks.

所有跟帖： 

• 嗬嗬，我聽了聽 -非文學青年- ♀ (912 bytes) () 05/26/2011 postreply 22:42:12

• 文青，你的IQ有180麽。 回複：嗬嗬，我聽了聽 -beautifulwind- ♀ (0 bytes) () 05/26/2011 postreply 22:56:07

• 這裏還有一個obama上周的講話也是一個笑話開始的 -非文學青年- ♀ (298 bytes) () 05/26/2011 postreply 23:06:38

• 回複：這裏還有一個obama上周的講話也是一個笑話開始的 -維文- ♀ (41 bytes) () 05/26/2011 postreply 23:12:43

• 我是這樣理解的。。。 -學英語3151- ♀ (204 bytes) () 05/27/2011 postreply 04:30:03

• 問的好 -hahaman- ♀ (0 bytes) () 05/27/2011 postreply 08:05:05

• 我覺得文青是對的。 注意either。。。or。。。 -beautifulwind- ♀ (0 bytes) () 05/27/2011 postreply 10:03:19

• 視頻:President Obama speech in Westminster Hall -紓珈- ♀ (184 bytes) () 05/27/2011 postreply 10:06:29

• Because ... -美壇奇葩- ♂ (18814 bytes) () 05/27/2011 postreply 19:58:10

• Bar Jokes -美壇奇葩- ♂ (68 bytes) () 05/27/2011 postreply 20:06:30

• 奇葩，I beg to differ -非文學青年- ♀ (182 bytes) () 05/27/2011 postreply 20:21:55

• Not a bar joke -學英語3151- ♀ (173 bytes) () 05/28/2011 postreply 21:33:58

• agree. 絕對不是behind bars裏的bar -hahaman- ♀ (0 bytes) () 05/29/2011 postreply 00:09:56