If you were to explain to an alien race what cinema is and provide only one movie as an example, what would it be? Bonus points for providing an explanation of why.

As for me, I would probably opt for "2001: A Space Odyssey".
This isn't necessarily a comfort film; it's an heavy, thought-provoking piece that took me several viewings to fully appreciate its magnitude, and now, as far as I am concerned, it stands as the most ambitious film of all time.

Stanley Kubrick succeeded in bringing to the screen a science fiction masterpiece that is both cosmically expansive and profoundly introspective, with the central subject being nothing less than the evolution of consciousness itself. The opera works on multiple levels, but its most powerful allure is subliminal, tapping into the subconscious, evoking ancestral memories, and foreshadowing a universal destiny. Even today, nearly 60 years later, despite immense strides in technology and special effects, one cannot help but be astounded by how far ahead of his time the director was both technically and in terms of screenplay.
"2001: A Space Odyssey" embodies the very essence of cinema for me.