"A friend, an intelligent lapsed Jew who observes the Sabbath for reasons of cultural solidarity, describes himself as a Tooth Fairy Agnostic. He will not call himself an atheist because it is in principle impossible to prove a negative. But "agnostic" on its own might suggest that he though God's existence or non-existence equally likely. In fact, though strictly agnostic about God, he considers God's existence no more probable than the Tooth Fairy's.
Bertrand Russell used a hypothetical teapot in orbit about Mars for the same didactic purpose. You have to be agnostic about the teapot, but that doesn't mean you treat the likelihood of its existence as being on all fours with its non-existence.
The list of things about which we strictly have to be agnostic doesn't stop at tooth fairies and celestial teapots. It is infinite. If you want to believe in a particular one of them -- teapots, unicorns, or tooth fairies, Thor or Yahweh -- the onus is on you to say why you believe in it. The onus is not on the rest of us to say why we do not. We who are atheists are also a-fairyists, a-teapotists, and a-unicornists, but we don't have to bother saying so."
From "A Challenge to Atheists: Come out of the Closet", by Richard Dawkins.