"'I do set my bow in the cloud." Genesis 9: 9-15
After the flood, God demonstrates remose and makes a deal with the creation, promising never to drown it again. He even puts the rainbow in the sky to remind himself of his promise. Now, whenever God gets angry, he looks at the rainbow and says: 'Oh, yeah.... That's right. I promised not to drown my creations again.' (It may seem like that last line was meant as a joke, but the idea that a rainbow is nothing but a giant post-it-note in the sky is pretty much Gen 9:15 verbatim)
The trouble with this verse is that rainbows occur naturally when light is refracted through water. They've existed for as long as light and water have existed. Are we to believe that there were no rainbows until after the flood? Did the flood change the very nature of light and water?
It won't surprise you to learn that traditional Judaism offers two answers, and, naturally, the nuttier one is more popular.
Nutty Torah-True Explanation: (more popular)
Before the deluge, some kind of vapor or cloud canopy surrounded the earth. 'Before the Flood, the clouds in the sky were thick and dense, obscuring the light of the sun. The Flood, which cleansed and purified the earth, also refined the clouds and made it possible for the rainbow to be observed, a phenomenon caused by the sun's rays' [Source] In other words, before the flood Earth's atmosphere with similar to the atmosphere of Venus. How did the people who lived before the flood survive the super high tempertures that are the physical result of such super-greenhouse conditions? The nutters don't say.
Rational Torah-True Explanation (less popular)
Samson Rephael Hirsch says, simply and specifically that the verses in Genesis shouldn't be construed to suggest that Noah's rainbow was the first rainbow in history. He bases himself on the Ramban, who writes: "As for us, we have no choice but to accept the opinion of the Greeks who maintain that the rainbow is an entirely natural phenomenon caused by the sun's rays striking the moist air after a rainfall. Any vessel of water that is placed in the sunlight will also produce a rainbow-like effect....The term "My rainbow" implies that it had existed earlier. Therefore, we explain the verses to mean that the rainbow that I placed in the clouds from the time of creation shall from this day forward constitute the sign of the covenant between Me and you, for whenI shall see it I will recall that a covenant of peace exits between Me and you…(commentary to 9:12-17).]
The Ramban, you'll notice, is not adverse to reinterpreting a verse based on the teachings of science. Unlike too many of my ridiculous commenters, he does not ask us to accept teachings which conflict with empirical evidence.