I just watched a movie about a magic tractor.

This joke expertly employs wordplay to deliver its chuckle. The setup about a "magic tractor" primes us for something extraordinary, leading us to anticipate a fantastical transformation. When the punchline arrives, "it went down the road and turned into a field," the humor lands because "turned into" has been cleverly twisted. We expect the magical meaning of changing form, but instead, the joke delivers the completely ordinary action of a vehicle simply changing direction and entering a field.

The real-world context here is straightforward: tractors are workhorses of agriculture. They are designed to travel from one location to another, often on a road, before entering the very fields they are meant to cultivate. The mundane reality of a tractor's journey, making a turn into its workspace, is precisely what makes the punchline so effective in subverting the expectation of magic.

It's a delightful example of how language can play tricks on our minds, setting up one interpretation only to reveal a much more literal, and thus unexpectedly funny, truth. The cleverness lies in the simple, everyday action being presented as if it were a grand, magical feat, making us smile at the linguistic sleight of hand.