(obligatory Douglas Adams reference)
"You get this bath, see? Imagine you've got this bath. And it's ebony. And it's conical."
"Conical?" said Arthur. "What sort of ..."
"Shhh!" said Ford. "It's conical. So what you do is, you see, you fill it with fine white sand, all right? Or sugar. Fine white sand, and/or sugar. Anything. Doesn't matter. Sugar's fine. And when it's full, you pull the plug out ... are you listening?"
"You pull the plug out, and it all just twirls away, twirls away you see, out of the plughole."
"You don't see. You don't see at all. I haven't got to the clever bit yet. You want to hear the clever bit?"
"Tell me the clever bit."
"I'll tell you the clever bit."
Ford thought for a moment, trying to remember what the clever bit was.
"The clever bit," he said, "is this. You film it happening."
"Clever," agreed Arthur.
"You get a movie camera, and you film it happening."
"That's not the clever bit. This is the clever bit, I remember now that this is the clever bit. The clever bit is that you then thread the film in the projector ... backward!"
"Yes. Threading it backward is definitely the clever bit. So then, you just sit and watch it, and everything just appears to spiral upward out of the plughole and fill the bath. See?"
"And that's how the Universe began, is it?" said Arthur.
"No," said Ford, "but it's a marvelous way to relax."