When you think about flying, it's nuts really. Here you are at about 40,000 feet, screaming along at 700 miles an hour and you're sitting there drinking Diet Pepsi and eating peanuts. It just doesn't make any sense.

Letterman’s joke hinges on defamiliarization: he describes commercial air travel in blunt, physical terms—altitude, speed, and the violence implied by “screaming along”—then juxtaposes it with the banal rituals of passenger comfort (soft drink and peanuts). The humor comes from exposing the mismatch between the extraordinary technological feat of flight and how quickly humans normalize it, treating it like an ordinary snack break. Implicitly, the line also pokes at modernity’s ability to make the miraculous feel routine, and at the thin veneer of calm that masks the underlying improbability (and latent anxiety) of being propelled through the sky in a pressurized tube.