Filmmakers are by necessity also film nerds, so it should come as no surprise that they often take the opportunity to insert an obscure reference to one of their favourite movies into whichever film they are working on.
Here are some of the subtlest nods that ever snuck across the big screen.
1. Die Hard with a Vengeance references Pulp Fiction
John McClane (Bruce Willis) tells Zeus Carver (Samuel L Jackson) that he spent his suspension from the police "smokin' cigarettes and watchin' Captain Kangaroo" – a quote from 'Flowers on the Wall', a song Willis sings along to in Pulp Fiction.
2. Captain America: The Winter Soldier references Pulp Fiction
The gravestone for Samuel L Jackson's Nick Fury quotes Ezekiel 25:17 – "The path of the righteous man..." This is the same quote that Jackson's character Jules Winnfield famously recites throughout Pulp Fiction.
3. Charlie and the Chocolate Factory references Batman
Charlie's dad works in a factory where Smilex toothpaste is produced. Smilex is also the name of the Joker's deadly laughing gas in Tim Burton's earlier film, Batman.
4. Phantom Menace references 2001: A Space Odyssey
The EVA Pod from the Stanley Kubrick classic can be seen lying forgotten and unloved in the junkyard next to Watto's Shop in Mos Espa.
5. Boogie Nights references A New Hope
Don Cheadle's Buck Swope desperately tries to upsell the TK-421 modified stereo in Boogie Nights. TK-421 just happens to be the designation of the stormtrooper whose armour Luke steals in the original Star Wars film.
6. The Social Network references Fight Club
Mark Zuckerberg (Jesse Eisenberg) uses a fake Facebook profile to cheat on an art exam (don't ask us how that works) in David Fincher's film. The profile's username is 'Tyler Durden', the counterfeit character Brad Pitt famously played in Fincher's Fight Club.
7. The Incredibles references Die Hard with a Vengeance
Samuel L Jackson voices Frozone, who is held at gunpoint by a cop when he is forced to break into a jewellery store. This parodies Jackson's scene in the third Die Hard movie, substituting a water dispenser for a ringing telephone.
8. The Force Awakens references Raiders of the Lost Ark
JJ Abrams's Star Wars comeback recycled the sound of the rolling boulder that famously chased Harrison Ford's Indiana Jones – you can hear it when the Rathtar rolls after Ford's other iconic character, Han Solo.
9. MCU Phase Two references The Empire Strikes Back
Someone loses an arm or hand in every movie in Marvel's Phase Two (that's Iron Man 3, Thor: The Dark World, Captain America: The Winter Soldier, Guardians of the Galaxy, Avengers: Age of Ultron and Ant-Man). Marvel Studios president Kevin Feige has admitted that what began as unintentional became a deliberate homage to Luke Skywalker losing his hand in Star Wars' second instalment.
10. Scream references Nightmare on Elm Street
Director Wes Craven appears in Scream as the school janitor, who wears a striped sweater very similar to Freddy Krueger's, the killer from another Craven classic, Nightmare on Elm Street. That gardener's name? It's Fred.
11. Out of Sight references From Dusk Till Dawn
When US marshall Karen Sisco (Jennifer Lopez) sees a mugshot of career criminal Jack Foley (George Clooney) on TV, she remarks that it "doesn't even look like him". And she's right... sort of. It's actually a picture of another criminal, Seth Gecko, who Clooney played in From Dusk Till Dawn.