McDonald's Corporation is the world's largest chain of hamburger fast food restaurants, serving more than 58 million customers daily. In addition to its signature restaurant chain, McDonald’s Corporation held a minority interest in Pret A Manger until 2008, was a major investor in the Chipotle Mexican Grill until 2006, and owned the restaurant chain Boston Market until 2007. A McDonald's restaurant is operated by either a franchisee, an affiliate, or the corporation itself. The corporation's revenues come from the rent, royalties and fees paid by the franchisees, as well as sales in company-operated restaurants. McDonald's revenues grew 27% over the three years ending in 2007 to $22.8 billion, and 9% growth in operating income to $3.9 billion. McDonald's primarily sells hamburgers, cheeseburgers, chicken products, french fries, breakfast items, soft drinks, shakes, and desserts. In response to changing consumer tastes, the company has expanded its menu to include salads, wraps and fruit.