Donald Trump Jr. posted a photo of himself outside Buckingham Palace -- and other social media users asked what he was even doing there.

The president's adult children joined him and first lady Melania Trump, as well as some White House staffers, on a state visit to the United Kingdom, where they met Queen Elizabeth II and prime minister Theresa May.

But their presence on the trip raised many questions, especially after the president's eldest son -- who helps run the family business while Trump serves in the White House -- posted a photo of himself after meeting the queen.