Certain foods, holidays, or customs seem like they must be how everyone in the world goes about their lives until you leave the country or region you grew up in and learn that not everyone has to take Massachusetts history class in high school, have a "Boston Tea Party" field day, or takes the day off when the Red Sox win. Don't ask anyone who isn't American how their Thanksgiving was unless you want to embarrass yourself, and not everyone in the world thinks a peanut butter and marshmallow fluff sandwich should count as a meal.
So, when a Reddit user asked, "What is something you grew up around not realizing was only popular in your country/region?" people were ready to share the things from their childhood that they thought were universal until they left their hometown.
Bagged milk. - Gubble_Buppie