I was impressed with myself I got this joke.

Jokes from the G+hole - Boing Boing:

"Werner Heisenberg is pulled over for speeding. When the state trooper asks 'Do you have ANY idea how fast you were going?', Heisenberg just smiles and says, 'No, but I know where I was!'—Laconia Laconia"

The Heisenberg Uncertainty Principle [from physics] explained here...

...and from the comments, this winner, especially given the next Foreign Service post we'll end up at:

"I sat on the train this morning opposite a stunning Thai girl.
I kept thinking to myself, please don't get an erection,
please don't get an erection...
but she did."

...and if you don't get that joke, click here.