This, of course, is a perennial favorite:
Although there are many others that will also do the job. For example, this one:
Renee Descartes walks into a bar. The bartender asks if he wants a drink. “I think not!” he says, and disappears.
Or another classic:
Heisenberg is driving along the highway, when he sees a cop car behind him. He pulls over, and the policeman gets out and comes over to the car. “Do you know how fast you were going, sir?” he asks.
“No,” says Heisenberg. “But I know exactly where I am!”
If the person you are talking to laughs at any of these jokes in a non-just-being-polite manner, you know you have a fellow nerd and new friend. So begins a lifetime (or half an hour, depending on whether or not you like them) discussing the new 70-minute Phantom Menace review and building model starships together.