The Celestial Teapot is a hypothetical teapot dreamed up by Bertrand
Russell. It demonstrates the absurdity of asserting that just because
a lack of evidence can't disprove the existence of something, it is
therefore equally likely to exist.
The gist is that someone can claim there is a perfectly formed teapot
orbiting the sun in outer space, quite separate to all the teapots safely
orbiting the sun from the comfort of Earth's surface. This person can
fervently insist the teapot exists, although naturally it's too small
to be seen with any telescope or analysed with any spectroscope.
We cannot disprove the existence of things - ghosts, goblins or gods, for
instance - just because there is a lack of evidence for their existence.
The point of the teapot story is that this doesn't mean the mythical
entities in question are just as likely to exist as they are to be works
It's perfectly rational to suggest that a phenomenon that is very
unlikely to have appeared by any natural means (ie, one that could
only have appeared by supernatural means), that no one can prove the
existence of, is almost certainly non-existent. Therefore, the burden
of proof lies upon the people claiming that it exists, not the people
claiming it to be a work of fiction.
The celestial teapot is similar to the paradoxical invisible pink unicorn,
the Flying Spaghetti Monster, and the dragon in Carl Sagan's garage,
the latter of which can be found in his book The Demon-Haunted World:
Science as a Candle in the Dark. I'd also recommend reading the parable
of the world where all the lost Biros end up, from that wholly remarkable
book, The Hitchhiker's Guide to the Galaxy.