From the immortal work of Douglas Adams' 'The Hitchhiker's Guide to the Galaxy'
The Babel fish is small, yellow and leechlike, and probably the oddest thing in the Universe. It feeds on the brainwave energy received not from its own carrier but from those around it. It absorbs all unconscious mental frequencies from this brainwave energy to nourish itself with. It then excretes into the mind of its carrier a telepathic matrix formed by combining the conscious thought frequencies with nerve signals picked up from the speech centers of the brain which has supplied them. The practical upshot of all this is that if you stick a Babel fish in your ear you can instantly understand anything said to you in any form of language.
The speech patterns you actually hear decode the brainwave matrix which has been fed into your mind by your Babel fish.
Now it is such a bizarrely improbable coincidence that anything so mind-bogglingly useful could have evolved purely by chance that some thinkers have chosen to see it as a final and clinching proof of the nonexistance of God.
The argument goes something like this: "I refuse to prove that I exist," says God, "for proof denies faith, and without faith I am nothing."
"But," says Man, "the Babel fish is a dead giveaway, isn't it? It could not have evolved by chance. It proves you exist, and so therefore, by your own arguments, you don't. QED."
"Oh dear," says God, "I hadn't thought of that," and promptly vanishes in a puff of logic.
Now insert any irreducible complexity claim fom the Intelligent Design (ID) department instead of the Babel fish, and there you have it.