What are things you can only really understand after you experience it

I notice a lot of "realization" posts in my various social medias. People who have kids and suddenly understand what they already were told. In this sub, I see a lot of people understanding aging as they experience it. It's nothing particularly groundbreaking, but the understanding comes like an epiphany. So far I have "Parenting" "aging" "addiction" and "sex" as things that can only be really understood by experiencing it. You can't really understand by reading, listening, or watching others.