"It is worth repeating at this point the theories that Ford had come up with, on his first encounter with human beings, to account for their peculiar habit of continually stating and restating the very very obvious, as in "It's a nice day," or "You're very tall," or "So this is it, we're going to die." His first theory was that if human beings didn't keep exercising their lips, their mouths probably shriveled up. After a few months of observation he had come up with a second theory, which was this--"If human beings don't keep exercising their lips, their brains start working."

Source: Douglas Adams (2009). “The Restaurant at the End of the Universe”, p.100, Pan Macmillan