What’s a “school lesson” that a book has taught you?

By school lesson I mean something that you could've learned in the classroom growing up but for whatever reason never did.

For me, I'm on a trip in a vastly different time zone than where I live and was just calculating what time it was back home. The term modular popped into my head and I was able to easily and accurately calculate via a twenty-four hour clock whether my family would still be asleep. I learned about modular counting from Neal Stephenson's "Cryptonomicon". Not the only math-related lesson he's taught me, to be sure. Came dredged up outta no where, haha.

Wonder what y'all's are. I'm posting this with fiction in mind, though it doesn't have to be.