This is taken from the Forum at the webcomic Questionable Content, but it is actually a paraphrase of a piece that Antony Flew wrote in 1950 called Theology and Falsification.

Anna and Kate are walking down the street when they come to an empty lot.

“Look at that beautiful garden!” cries Anna, “It must be tended by a particularly skillful gardener!”

“What are you talking about?  It’s an empty lot full of weeds!  It couldn’t possibly be tended by a gardener.” is Kate’s reply.

“He must tend it to grow that way.”

“I have never once seen a gardener there.”

“You must have simply missed him.  He must come only at night, when you don’t walk by here.”

So Anna and Kate decide to watch the garden.  They keep watch for days, and don’t spot a gardener.

Kate shrugs and says, “I guess there’s no gardener.”

“He must be invisible.”

“… invisible?”

“Yes, that’s why we couldn’t see him.”

So Anna and Kate build a fence.  When Anna suggests he might be able to fly, they put a net over the fence.  When Anna suggests he might be very small, they put a solid dome over the entire plot.  Then Anna suggests he might be intangible.  Kate gets fed up.

“You’ve got an invisible, intangible, flying gardener whose garden looks exactly like there isn’t a gardener.  What’s the difference between that and a gardener who doesn’t exist?”

Now before you non-atheists get all “hurr, hurr, Flew converted!” on me, please know what you’re talking about before you choose to speak about it.  Read his Wikipedia entry (link above), and be prepared to amend what you think you know (or gloss over the truth and make something up, whatever your choice is).   From a BBC Interview:

Q What view do you take of what is happening in America – where presumably you’re being hailed now as… one of them?
A Well, too bad (laughs). I’m not ‘one of them‘.