"A trade dress feature is 'functional,' and therefore not protectable, if it is 'one which competitors would have to spend money not to copy but to design around. . . . It is something costly to do without (like the hood [of a car]), rather than costly to have (like the statue of Mercury [on the hood of a Rolls Royce]).' Put another way, a functional feature is one that 'would be found in most or all brands of the product even if no producer had any desire to have his brand mistaken for that of another producer.' It is a feature, such as the oval shape of a football, 'that competitors would find necessary to incorporate into their product in order to be able to compete effectively.'"
- Computer Care v. Service Systems Enterprises, 982 F.2d 1063, 1071 (7th Cir. 1992) (citations omitted).