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： 

• 呵呵，我听了听 -非文学青年- ♀ (912 bytes) () 05/26/2011 postreply 22:42:12

• 文青，你的IQ有180么。 回复：呵呵，我听了听 -beautifulwind- ♀ (0 bytes) () 05/26/2011 postreply 22:56:07

• 这里还有一个obama上周的讲话也是一个笑话开始的 -非文学青年- ♀ (298 bytes) () 05/26/2011 postreply 23:06:38

• 回复：这里还有一个obama上周的讲话也是一个笑话开始的 -维文- ♀ (41 bytes) () 05/26/2011 postreply 23:12:43

• 我是这样理解的。。。 -学英语3151- ♀ (204 bytes) () 05/27/2011 postreply 04:30:03

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

• 我觉得文青是对的。 注意either。。。or。。。 -beautifulwind- ♀ (0 bytes) () 05/27/2011 postreply 10:03:19

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

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

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

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

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

• agree. 绝对不是behind bars里的bar -hahaman- ♀ (0 bytes) () 05/29/2011 postreply 00:09:56