What is the Marines and Crayons joke?

It’s funny because jokes about eating crayons are occasionally made by Marines as well. Much of it stems from the tasks that Marines are frequently assigned, which call for the ardent destruction of property and the people who protect it in a manner similar to a toddler destroying a room—and the fact that Marines are exceptionally skilled at this.
In addition, a great deal of the duties assigned to the Marines are rather simple tasks. When people from other services refer to Marines as crayon-eaters, they usually mean that they behave like that destructive kid again—that is, they do stupid things with great passion.

The joke “We all know Marines eat crayons, but what’s their favorite flavor of crayon? Crayonberry” plays on a stereotype and wordplay for comedic effect. Firstly, there’s a humorous stereotype or running joke suggesting that Marines, known for their toughness and no-nonsense attitude, are so straightforward that they would eat crayons, an item typically associated with children and non-edibility. This stereotype is used here for light-hearted humor, not to be taken seriously.
The punchline “Crayonberry” is a play on words, combining “crayon” and “cranberry.” Cranberries are a common fruit flavor, and by blending it with the word “crayon,” it humorously suggests that if Marines were to have a favorite crayon flavor, it would be something akin to a real fruit flavor, but still within the realm of crayons.