Does your guacamole turn brown before your guests arrive? Here is a solution: smooth out the guacamole’s  surface then press plastic wrap directly onto the dip, pressing lightly, and refrigerate. Avocados brown when exposed to oxygen. This sealing the surface slows down oxidation so it will stay green longer. If you top your guacamole with a thin layer of water or lime juice, this will block out the air even more. Yum!