Joke about a magical tractor, I dont get it
The joke is using two different meanings of "turned into." The first is that the tractor drove off of the road and turned a corner into a field - a common activity for a tractor to do. "The car turned into the driveway," "I turned left into Main Street," etc.
The second is saying that since this was a magical tractor, actually it physically turned/transformed into a field. "The magician turned his hat into a rabbit," "The caterpillar turned into a butterfly."
(Check the different meanings here: https://idioms.thefreedictionary.com/turn+into )