The definition of 'play' is elusive

When I hear people say, "They're just playing" I need to remind myself that not everyone is aware of the research about play, what play involves, and the important role that playing has throughout our lives. Hopefully, the visuals and quotes from @aredotna and this excerpt from the American Academy of Pediatrics report on play help shed some light on what play is, as well as its importance:

"However, there is a growing consensus that it (play) is an activity that is intrinsically motivated, entails active engagement, and results in joyful discovery. Play is voluntary and often has no extrinsic goals; it is fun and often spontaneous. Children are often seen as actively engaged in and passionately engrossed in play; this builds executive functioning skills and contributes to school readiness.

Play often creates an imaginative private reality, contains elements of make believe, and is nonliteral. Play is not frivolous; it enhances brain structure and function and promotes executive function (i.e., the process of learning, rather than the content), which allows us to pursue goals and ignore distractions."

• 2018 clinical report on play by the American Academy of Pediatrics