The '9+10=21' meme originated from a viral 2013 Vine video featuring a young boy being asked a math question by his older brother. When prompted to solve '9 plus 10,' the boy confidently answers '21,' leading his brother to jokingly retort, 'You stupid.' The clip became an instant internet classic due to the boy's deadpan delivery and the sheer absurdity of the incorrect answer. Over the years, it has evolved into a symbol of 'internet math' and is often referenced in '21' day celebrations on September 10th or used as a reaction to highlight someone making a simple error with total confidence.