Because sometimes one movie just isn't enough, some films dare to tease another work of cinema inside of themselves.
The movie-within-a-movie gambit has primarily allowed filmmakers to poke fun at the industry, themselves, audiences, or all of the above, by rustling up a fake, fictional film that doesn't actually exist.
And while most of these films within films are mere throwaway jokes about how terrible they'd probably be in real life, every so often a fake movie emerges that audiences would actually love to watch.
Perhaps the fakeness was persuasive enough that people actually assumed it was a movie, or the premise simply proved tantalising enough that fans hoped they could somehow will it into existence.
Sadly none of these 10 movies have ever gone before cameras and made it to our eyeballs in earnest, so we're left with these mere morsels instead: fake trailers, single scenes, or perhaps only the suggestion of what the film could've been.
From certifiably insane sequels to films we never thought we needed and everything else in between, it's a damn shame these movies within movies don't genuinely exist...
10. Angels With Filthy Souls - Home Alone
Is there any better testament to a film-within-a-film's quality than most people believing it to be an actual, real movie?
For younger viewers in particular, this was certainly the case with Home Alone's Angels with Filthy Souls - an apparently decades-old black-and-white gangster film memorably watched by Kevin McCallister (Macaulay Culkin) while left to his own devices during the movie.
There isn't a person alive who's seen Home Alone and doesn't remember Johnny (Ralph Foody) pumping Snakes (Michael Guido) with a hail of lead before savagely quipping, "Keep the change, ya filthy animal!" It just isn't possible.
But of course, the scene was shot specifically for Home Alone, as a parody of the 1938 crime film Angels with Dirty Faces, with director Chris Columbus even going to the trouble of shooting it on black-and-white negative film that would help give it a period-accurate look.
It certainly looks like an absolute blast, enough that Home Alone 2 rehashed the gag to still-amusing effect with the sequel, Angels with Even Filthier Souls.
Between the deliciously hard-boiled dialogue and Ralph Foody's gloriously scenery-chewing performance as Johnny, what's not to like?