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Star Wars fan @JessTrouper tweeted a photograph Tuesday that shows a young Hamill on the set of Star Wars with his jaw gaping open.

SALT LAKE CITY — Mark Hamill has been waxing nostalgic on Twitter this week, particularly about how he learned that Luke and Leia were siblings.

A Star Wars fan @JessTrouper tweeted a photograph Tuesday that shows a young Hamill on the set of Star Wars with jaw gaping open.

“I’m somewhat curious about this picture,” @JessTrouper tweeted. “What happened here?”

Hamill responded simply, “The moment I realized I kissed my sister.”

The exchange prompted another question from a curious fan.

“I've kinda always wondered,” @Vampirebunbun tweeted. “At what point did they let you all know about Luke and Leia being siblings? And what were yours and Carrie's reactions?”

Hamill responded with a funny sample of the exchange he had with Carrie Fisher following the revelation that they were siblings.

Hamill tweeted, “Me: Wait a minute...If Luke is Princess Leia's brother, does that make me royalty?

“Carrie: (immediately w/ no hesitation) NO!!!

“Me: OK-relax! Just asking... (So began a yrs-long tease of mine) You sure? LORD Vader-QUEEN Amidala & I'm the only commoner?

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Hamill described Fisher’s reaction to his inquiries with a frowning emoji and added the hashtag #TrueStory to his tweet.

The exchange feels pretty on brand for both Hamill and Fisher’s "Star Wars" personas, and fans have been responding in kind.

“Amidala was a queen, but it was an elected position and she was no longer queen when Luke and Leia were born. Leia is a princess because she was adopted by the Queen and Prince Consort of Alderaan. Sorry, farm boy,” @datadivajf tweeted.

“You are the Prince of Moisture farming,” @kernicus tweeted.

“She always will be our princess,” @maka_gh tweeted.