The idea of storing preserved foods in the middle-class American house was common prior to the advent of refrigeration. Canned fruits, meats, and vegetables could be safely stored for months in cool basement rooms or external root cellars. However, the creation of new types of packaging and an increase in the number of grocery outlets in the early twentieth century reduced most American's need to store large quantities of food in their homes. When America's participation in World War I and World War II diminished the availability of tin and certain parts of the food supply, however, Americans relied on their own provisions once again. This meant that twentieth century houses required specific types of spaces that could preserve foods for months at a time. Meats, fruits and vegetables held in glass canning jars required dark spaces to maximize their usefulness. Root vegetables and fruits such as apples or pears required humid but ventilated space to prevent them from molding or withering. Homebuilders and homeowners commonly modified existing basement and aboveground rooms to provide both types of conditions.