To explain --- since every piece of matter in the Universe is in some way affected by every other piece of matter in the Universe, it is in theory possible to extrapolate the whole of creation --- every sun, every planet, their orbits, their composition and their economic and social history from, say, one small piece of fairy cake.
The man who invented the Total Perspective Vortex did so basically in order to annoy his wife.
Trin Tragula --- for that was his name --- was a dreamer, a thinker, a speculative philosopher or, as his wife would have it, an idiot.
And she would nag him incessantly about the utterly inordinate amount of time he spent staring out into space, or mulling over the mechanics of safety pins, or doing spectrographic analyses of pieces of fairy cake.
``Have some sense of proportion!'' she would say, sometimes as often as thirty-eight times in a single day.
And so he built the Total Perspective Vortex --- just to show her.
And into one end he plugged the whole of reality as extrapolated from a piece of fairy cake, and into the other end he plugged his wife: so that when he turned it on she saw in one instant the whole infinity of creation and herself in relation to it.
To Trin Tragula's horror, the shock completely annihilated her brain; but to his satisfaction he realized that he had proved conclusively that if life is going to exist in a Universe of this size, then the one thing it cannot afford to have is a sense of proportion.
The door of the Vortex swung open.
From his disembodied mind Gargravarr watched dejectedly. He had rather liked Zaphod Beeblebrox in a strange sort of way. He was clearly a man of many qualities, even if they were mostly bad ones.
He waited for him to flop forwards out of the box, as they all did.
Instead, he stepped out.
``Hi!'' he said.
``Beeblebrox ...'' gasped Gargravarr's mind in amazement.
``Could I have a drink please?'' said Zaphod.
``You ... you ... have been in the Vortex?'' stammered Gargravarr.
``You saw me, kid.''
``And it was working?''
``And you saw the whole infinity of creation?''
``Sure. Really neat place, you know that?''
Gargravarr's mind was reeling in astonishment. Had his body been with him it would have sat down heavily with its mouth hanging open.
``And you saw yourself,'' said Gargravarr, ``in relation to it all?''
``Oh, yeah, yeah.''
``But ... what did you experience?''
Zaphod shrugged smugly.
``It just told me what I knew all the time. I'm a really terrific and great guy. Didn't I tell you, baby, I'm Zaphod Beeblebrox!''
His gaze passed over the machinery which powered the vortex and suddenly stopped, startled.
He breathed heavily.
``Hey,'' he said, ``is that really a piece of fairy cake?''
He ripped the small piece of confectionery from the sensors with which it was surrounded.
``If I told you how much I needed this,'' he said ravenously, ``I wouldn't have time to eat it.''
He ate it.