Outlanders
My older brother told my little brother (about 5 at the time) that humans didn’t originate on planet Earth, that we had to abandon our planet because our star was going supernova, and we lost our technology in the process.
Very unoriginal sci-fi plot, but the kid was 5 so he was somewhat suspicious, but still willing to believe. He came to me to ask for confirmation, because I wasn’t the type of person to try to trick him. I was going to tell him it was a lie, but thought it might be funny, so I told him it was true and forgot about it.
When my little bro was 16 he told us that he believed that for years and convinced a bunch of his school friends it was true. He was so mad. It was hilarious. Story credit: Reddit / watermusic