Every afternoon, a perfessor of theoretical physics comes into the university bar, sits in the next-to-last seat, turns to the last seat -- which is empty -- and asks a woman who isn't there if he can buy her a drink.

The barmaid is accustomed to weird university types, so she just silently shrugs at his antics.

As Valentine's Day arrives, and the persistent perfessor makes a particularly heart-wrenching plea into empty space. Curiosity gets the better of the barmaid, so she asks "Surely you notice
there is never a woman sitting in that last stool. Why do you persist in 'hitting on' empty space?"

The physicist replies "According to quantum physics, 'empty space' is never truly empty. Virtual particles come into existence and vanish all the time. You never know when the proper wave function will collapse exactly the right configuration of particles, and a woman might suddenly appear there."

The barmaid raises her eyebrows. "Intriguing. Or you could just offer to buy a drink to one of the dozens of horny gals cruising here every evening. You never know... she might say "YES!!!"".

The physicist knowingly laughs "Yeah, right. And how likely is that to happen?"