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On January 20, as Donald Trump became the 45th President of the USA, wife Melania stole the limelight at his inauguration for looking somewhat stony-faced.

That was, until her husband turned around to look at her. She plastered a beaming smile across her face, but it instantly fell away when President Trump turned back around.

Viewers, noticing the drastic change in her facial expressions, immediately recorded the clip and started posting it on social media, where it's been shared hundreds of thousands of times.

The main question on everyone's lips is why her mood changed so much within the space of a few seconds. Watch it again now, and you might have your answer.

Most people have been focused on Melania Trump when watching the clip - but it's not her you should be looking at. Well, not really.

This time, focus your attention on Donald Trump's daughter, Ivanka, who is standing behind stepmother Melania and slightly to her right.

If you keep your eyes trained on Ivanka, you'll notice that while Melania grins wildly at her husband, Ivanka is really the one who appears to make eye contact with her father and exchanges a look and some words with him.

Trump and Ivanka both appear to raise their eyebrows at the same time, further suggesting that they are looking directly at each other.

It appears Trump, rather than turning around to say something to his wife, has looked straight past her to share the moment with his daughter instead.

Turn your attention back to Melania and it even looks like she's throwing some serious side-eye in Ivanka's direction and appears to mutter something under her breath.

Social media users have pointed out the theory and believe this is the real cause of Melania's frown.

To be honest, we wouldn't blame her if this were the real reason.

Expect '#InvisibleMelania' to be the next trending meme.

There has been speculation in the past that Ivanka will overshadow Melania as the real 'First Lady' of the USA.

She certainly had a higher profile than her stepmother during her father's campaign trail prior to the election in November 2016.

Donald Trump's stylish eldest child stood by him throughout every new negative story which came to light during his campaign, helping him to polish his image and change public perception.

(Image: Getty Images North America)

She has also taken an active role in politics in the run up to the inauguration, taking on some of the traditional First Lady duties.

Ivanka has served as a hostess for visits from dignitaries, met with the Japanese Prime Minister Shinzo Abe and executives from tech giants like Facebook and Apple.

She even had a meeting with former vice President Al Gore to discuss climate change in December.

(Image: Getty)

Melania, by contrast, has decided not to move to the presidential home in Washington, D.C. and is instead going to stay in her apartment in New York City with 10-year-old son Barron, who attends school there.