From the Houston Chronicle, which covered former president George W. Bush's speech at the Lyndon Johnson presidential library:

While delivering a serious message, Bush started with a couple of jokes, including one about presidents and their libraries.

"Former presidents compare their libraries the way other men may compare their, well ...," he said, to laughter. "Just wondering how LBJ would have handled that. He was a funny guy at times."

And here's the video, from KVUE-TV: