Mules! Hey, Joe, hear this? This guy wants to trade mules. Didn’t nobody tell you this is the machine age? They don’t use mules for nothing but glue no more.
– John Steinbeck
The Grapes of Wrath, Chapter 7. The used car dealer is mocking a dispossessed tenant farmer who wants to trade in his mules for a car. This is an example of the clash of two worlds, the old and the new, as the farmer’s traditional way of life is overtaken by science and technology.