In the comment section of a newspaper article reporting on the 2015 Nobel Prize for Physics, which was awarded for work on neutrino oscillation, I found the following joke:
"I'm sorry, we do not serve neutrinos", says the barman.
A neutrino walks into a bar.
What physical phenomenon does this actually allude to? The structure of the joke seems to imply some sort of time-reversal. I realize explaining a joke kind of defeats the purpose of the joke, but I'm just curious what the point is here.