Sour Green Apple Jello is a type of gelatin dessert that typically has a bright green color and a tangy, tart flavor reminiscent of green apples. It is usually made by mixing flavored gelatin powder with boiling water and allowing it to set in a mold or container until it becomes firm. The sourness in the Green Apple Jello comes from the addition of citric acid, which gives it a mouth-puckering taste that is balanced by the sweetness of the sugar in the gelatin.