One of the physicists replies, "We were just discussing stellar evolution. My colleague believes that within 5 billion years, the sun will run out of fuel and become a red giant. The Earth and everything on it would be incinerated, of course."
The drunk stands down, more at ease now. "God, that's a relief," he says, "I thought you said 5 million."
(I didn't make up that joke. In some form it might actually be older than me. I gave it a little touch-up to make it more cinematic, though.)