It’s a common scenario: You stop by the supermarket, purchase the ripest avocados and whip up some perfect guacamole. It tastes better than great, and you can’t wait to snack on it over the next few days. But when you go back for seconds, it’s turned an unappetizing shade of brown. As great as guacamole can be for game days and parties, plenty of folks decide against making a batch because of its short shelf life. But here’s some good news: There’s a way to keep this healthy snack fresh for days to come. The first solution to preserving guacamole begins with storage. Avocados contain polyphenol oxidase, an enzyme that, when exposed to oxygen, causes food to brown rapidly. (It’s also what causes apples and bananas to turn brown.) If oxygen is an avocado’s worst nightmare, then the best way to keep your guacamole fresh means protecting it from the air. After spooning your leftover guac into a …