Leonardo DiCaprio used a prop in one of his movies to give the huge plot twist away to viewers

Did you know the major plot twist in one of Leonardo DiCaprio's most famous movies is hinted at through one tiny prop?

Starring DiCaprio, Mark Ruffalo, Ben Kingsley, and Michelle Williams, Shutter Island is one of Martin Scorsese's highest-grossing films of all time.

The 2010 film follows the story of U.S. Marshal Edward 'Teddy' Daniels (DiCaprio) as he travels to a hospital which houses the criminally insane. After arriving on the island alongside his partner Chuck Aule (Ruffalo) the pair set about investigating the disappearance.

However, things soon take a pretty unexpected turn. Watch the trailer below if you're unfamiliar with the film:

From the get-go it's pretty obvious that all is not as it seems with Teddy, who is frequently plagued by flashbacks throughout the film before a mind-bending final twist is revealed.

In honour of DiCaprio's 50th birthday today (11 November), we're revisiting the clever way in which the actor was able to hint at Teddy's fate throughout the film. Spoiler warnings ahead.

What happens at the end of Shutter Island?

Towards the end of the film we learn that 'U.S. Marshal Edward Daniels' doesn't actually exist, with Teddy ultimately being revealed to be a patient named Andrew Laeddis.

Andrew has been incarcerated at the institution for killing his wife after she drowns their children. Meanwhile his work partner 'Chuck Aule' is actually revealed to be Andrew's psychiatrist Dr Lester Sheehan.

The 'missing patient' narrative was also something Teddy came up with to help him deal with the trauma. The staff decide to go along with it to see if it will help him accept reality, but it doesn't work and he's given a lobotomy instead.

Pretty chilling, right?

Anything look suspicious here? (Paramount Pictures)

How did Leonardo DiCaprio hint at Teddy's fate throughout the film?

A video uploaded by movie fan @imeugeneclark explains that a prop used in the film reveals that Teddy was a patient all along.

"Did you know that in Shutter Island a small detail reveals Teddy's true identity early on?" they explain.

"Of course, a big plot twist of the movie is finding out that Teddy Daniels isn't in fact a US Marshal but actually Andrew Laeddis, a patient at the mental sanatorium he is investigating.

"This twist is actually given away as every time that Teddy goes to smoke a cigarette, someone else lights it for him.

"This is because patients on Shutter Island are not permitted to carry lighters around with them."

This wasn't the only clue eagle-eyed viewers noticed either, with one person pointing out that Teddy's formal clothes are too big for him, while a third added the reason Teddy's 'partner' struggles to take off his holster before entering the facility is because he isn't used to it.

Clever, huh?