My little brother who's 7 was saying girls can’t be scientists, and my little sister who's 5 looked at him offended and said, “Princess Bubblegum is a girl and she's a scientist, Jonny!” and he said, “oh yeah…ok nvm,” and they continued eating breakfast like nothing.

Cleffairie on Tumblr (punctuation cleaned up)

The latest in a continuing series on immediate Bayesian updating in response to information. (Also viable as an example of an "unknown known," since he knew the counter-example but had not thought to apply it.)