Compatibility is key when it comes to relationships. Whether a relationship is romantic or platonic, you want to be able to connect with someone beyond the surface level. In some cases, you may even strive to share similar goals or interests.

Sometimes, we turn to different sciences or belief systems to help us determine compatibility. Take Zodiac signs for instance. If you're a Gemini, you might try to find a significant other who's an Aries or Leo.

There are probably hundreds of ways to convince yourself that your crush is "the one," but could liking olives be the indication? Here's a primer on the "olive theory."

What is the 'olive theory'?

The "olive theory" comes from the sitcom "How I Met Your Mother." It is first mentioned in the show's pilot. After characters Ted and Robin's first date, Ted returns to his shared apartment with friends (and couple) Lily and Marshall. Ted recounts his date to them, calling Robin "the future Mrs. Ted Mosby."

Through flashbacks, the audience sees how Robin checks off all of Ted's boxes for the ideal woman. But Ted saves the "best for last."

"Do you want these? I hate olives," says Robin during the date.

"She hates olives, awesome!" Marshall responds in the present, as Lily mentions the so-called "olive theory."

Back to the date flashback, Ted explains the theory is based on Marshall and Lily. Marshall hates olives, while Lily loves them. "In a weird way, that's what makes them such a great couple – the perfect balance," says Ted.

At first, the theory is similar to the age-old idea that opposites attract.

The theory's meaning, however, changes once Barney (who is a part of the characters' friend group) brings to light a shocking truth. While in a cab, Barney questions Marshall's distaste for olives.

"Two weeks ago, Spanish bar on 79th Street, dish of olives, you had some. What up?" Barney says.

Marshall then asks Barney to never tell anyone what he's about to say. On Marshall and Lily's first date, he ordered a Greek salad. Lily asked if she could have the olives in it and Marshall responded, "sure, I hate olives." It turns out Marshall actually likes olives and only offered them to Lily because he was very attracted to her.

Barney tells Marshall he shouldn't marry Lily, equating olives to new experiences, or to dating other women. But Marshall does not agree. "I'm marrying that girl," he says as Lily appears near the cab's open window.

"Lily, I like olives," Marshall says. "We'll make it work," she replies.

The true "olive theory" is about the give-and-take that comes with being in a relationship. When you love someone, you have to be willing to make sacrifices and compromises for them. In Lily and Marshall's case, it's about giving up olives. Marshall knows Lily loves olives. So, if he orders something that has olives, Marshall will always them to Lily, just to make her happy. This principle transcends olives and can be applied to anything.

Some users on social media have used the theory to show compatibility with their partner based on Ted's interpretation, while others follow the actual meaning.

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