Writing an opinion for the Supreme Court case Jacobellis v. Ohio, Justice Potter Stewart offered a definition of pornography that was also a lack of one: "I know it when I see it," he said.

You could say, if you wanted to be slightly creepy about it, the same thing about sandwiches. We have a basic idea of what a sandwich is: two slices of bread with some tasty stuff in between them, just as the Earl of Sandwich so ingeniously envisioned back in the 18th century. But get past those basics for form, and the specifics of function can make things … tricky. Is a hot dog a sandwich? What about a wrap? What about a gyro? Has an open-face "sandwich," with its single slice of bread, been lying to us this whole time?

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