I don't think Dr. Sagan would mind, but stop me if you've heard it already. Note that he never says, at least to my knowledge, that he's an atheist. He would sell fewer books and, understandably, reach fewer people who need to hear what he has to say. This analogy in his latest book, however, crystalizes his stance and is so good that I've used it several times since I've read it. The good doctor probably wrote it much better than I, but here's my version. I have embellished it, but only for dramatic purposes, as well as further to avoid copyright infringement.
I have a friend to tells me at work one day that he keeps a dragon in his garage. He can tell I'm dubious, so he offers to show it to me. I'll accept anything with ample evidence, so I accede and agree to have the dragon shown to me, and vice versa... and hopefully not toasted like a marshmellow.
Upon entering the garage my friend says, "There... that's all the proof you need." Looking around, however, I see only a disorganized assortment of tools and a puddle of toxic antifreeze on the floor.
"I'm sorry," I respond, "but I don't see any dragon here."
"Oh, well I forgot. He's an INVISIBLE dragon."
"I see, or that is, I don't see." Always prepared, I whip out an electronically enhanced pair of glasses. "Let's see if I can detect his heat in the infrared spectrum. Hmm... no, I can't see even normal body temperature, let alone the heat from an invisible firey breath."
"Oh, that's because this dragon doesn't give off heat. Reptiles are cold blooded, you know.... And his fire is room temperature."
At this response I remove the glasses and grab a can of oil from my friend's work-bench. I puncture a hole in the top and spread it evenly across the entire floor. After watching for a moment, I say, "I don't see any foot-prints."
"Didn't you know dragons can fly? He's hovering in the air right now."
Quickly I open the garage door, run outside, get into my car, and drive it into the garage. After stepping out of the car I say, "Is he still here?"
"How can he fit in here with the car?"
"Well," my friend says, "this dragon is noncorporeal. He is made of a material that does not conflict with normal matter, and he can therefore exist simultaneously in the same space with another object."
"Do you have any proof at all to show me that indicates the existence of this dragon?"
"Well, no. You just have to take it on faith. *I* know the dragon exists because... well, I just know!"
At this point I sigh and go have a beer.
I don't think that's how Sagan ended it, but I like it.