Many fast food and sit down restaurants serve burgers, fries, and patty melts with a "secret sauce". Secret sauce is usually a variation of thousand island dressing. The base is made with mayonnaise and ketchup, with other flavors added like mustard, paprika and garlic. The main difference is that thousand island dressing usually contains pickle relish, whereas burger sauce does not. I like to use a dash of pickle juice in my secret sauce to get the same flavor, but without the chunky texture.