Here's something I wrote a while ago (2006 or so).
What is science?
Science is starting with dirty window and washing it so you can see through it. At first it's a bit blurry and you can see something that looks like a truck engine. You say, "Well there's a truck out there." Then as you clean up more spots, you see more of the engine and some pipes. You think, "Oh, those must be part of the exhaust system."
Finally after a good portion of the window has been cleaned up, you can see that what you thought was a truck (based on what you thought was a truck engine and an exhaust system) really is a generator with cooling pipes. Joy fills you as you realize that you've been wrong and get excited at the possibilities, "Ha-ha! Maybe this is a refrigerator!" You clean up the window a bit more and see transformers and power stations. You revise yourself, "Oh, it's a power plant." Sure, you don't see all the connections and all the wiring, but it's definitely a power plant.
After some more work, you cleaned most of the window and figured out how the transformers and powers station work. You also see what looks like cables going between the transformers and power station. You hypothesize, "Well it would make a lot of sense if those were transmission lines because current has to flow between the two." Then you think a bit more, "Hmm, if this is a power station there ought to be stuff like gas tanks around if it's a thermal plant... Unless it's a hydroelectric power plant and there's a dam outside. Let's check for gas emission to see if it releases CO2!" So you measure CO2 levels and upon seeing the data, you think, "Hmm... there's definitely CO2 around so that could mean that it's a thermal plant... but there's also radiation present... maybe it's a nuclear plant... CO2 levels could be from a secondary system if it's a nuclear plant... or maybe it's a thermal plant and there's something that is radioactive around?"
This is how science works.
What is religion?
Religion is starting with a window with truck decals over it. You say, "Look there's a truck out there!" You run to the window to take a closer look, and you see some places in the decals where there are holes. You look through them and see a truck engine where the truck engine is on the decals. You think to yourself, "Cool, there's really a truck engine behind this, the decals are accurate." Then some guy throws a rock at the window and breaks the part where the exhaust pipes were. Everyone is shocked, "Someone dared to throw a rock in the window! It had a beautiful truck on it that made everyone happy."
You happen to be the guy in charge of patching things. As you get to the window, you look through the bigger hole left by the rock. You see something that looks like an exhaust pipe, but not really the way it was portrayed on the decal. You think, "Ah, well the decals were mostly accurate at least." And then the same guy as before throws another rock in the window because he wants to see what's behind the window. Oh boy, this time he broke a good chunk of it.
Everyone is shocked! Throwing rocks in windows! And the window with a truck on it at that! More shocking even is that you can now see that it's not a truck engine you were seeing but a generator with cooling pipes. Some people think, "Well this just means that trucks run on generators with cooling pipes." Other people claim it's the truck as depicted on the original decal and that those who see something else have hallucinations from the exhaust pipes of the truck. Other people say, "It's not a truck, it's a power plant, but that doesn't mean there can't be a truck in the power plant."
Then the scientist comes in and says, "Look we cleaned our window and look it's a power plant! You've got transformers, transmission lines, power stations, generators and all that jazz. We even figured out how they work for the most part. The window is still dirty in some parts but we'll get it cleaned up eventually." The religionist replies, "No, no, it's a truck. Look through my window!" The scientist takes a look and is puzzled for a while. Eventually, he realizes "Hmm that is weird... My window clearly shows a power plant... Ah I see the problem! You have decals over your window. That's why it looks like a truck. Let's take it off and you'll see the power plant." The religionist take offense, "No! My beautiful window, don't you touch it! It's a truck, you've just inhaled too much exhaust fumes!" and refuses to peek through your cleaned window out of fear of inhaling the same fumes you did.
The moderate religionist peaks through your window and say, "Well all those transformers and thingies ... I don't really understand them, but I guess it's just how truck works!" The agnostic takes a look and says, "Well your window is not fully cleaned yet, so we can't really say there isn't a truck out there. And even if it was completely cleaned, there could always be a truck hiding behind that big generator over there so we can't really say there is no truck there."
The scientist intervenes, "Hold your horses people. This is not a truck, it's a power plant! We didn't see any truck parts, skid marks, or oil leaks that can't be explained by a piece of equipment from the power plant. There's oil on the floor over there, and we don't know where it comes from yet from but we know it's not from a truck or a moving vehicle." The religionist retorts "Well how do YOU know! You thought it was a fridge before after all. What's to say that you're not again wrong this time? You can't say for sure that it's a power plant because that bit in the corner that's not cleaned could be a tire and the other part here could be a carburetor! You can't prove it's not a truck because you don't know everything!"
The agnostic chips in, "The religionist has a point. How can we really know there are no trucks out there, there could be one in the turbines?" The scientist admits, "Yeah well... Yeah I guess there could be one in the turbines since we haven't looked there. When turbine runs, none of our instruments works because of the interference." The atheist then replies, "Why the hell would there be a truck in the turbines? That doesn't make any damned sense!"
And then the philosopher comes in and busts everyone's balls by saying, "Well it's not because it's a power plant today that it won't be a truck tomorrow!"
* Carl Sagan (1995 ). "The Dragon In My Garage", The Demon Haunted World. ISBN 0-345-40946-9
* "Russell's Teapot", Wikipedia. Accessed 17 October 2009.
* "Invisible Pink Unicorn", Wikipedia. Accessed 17 October 2009.