CBS's David Letterman tried to squeeze some new information out of Bill Clinton Monday about whether former secretary of state Hillary Clinton will run for president in 2016.

The former president wouldn't bite.

"If she is running to your knowledge, blink twice," Letterman told Clinton.

"I blinked once!" said Clinton with a smile.

"You know that if I did know, I wouldn't tell you," Clinton later added. "But I honestly don't know, Chelsea doesn't know, and Hillary doesn't know."

Letterman joked, "Well, you could finish her book."