Child intuition?

Who has had a child say to them that there was a baby inside you or mentioned a brother/sister and was correct?😂 My 3yr old has literally never mentioned anything about babies before and tonight before bed drops the bomb on me that he plays hide&seek with his SISTER and she's in my belly....😬 He fell asleep a few minutes later so I know he was beyond tired but it was just sooo random!! Anyone have any experiences with this? Thanks!