One of my math professor (Canadian) once told us a story (~10 years ago) that the US border control asked him to "state and prove Rolle's theorem" after he had

One of my math professor (Canadian) once told us a story (~10 years ago) that the US border control asked him to "state and prove Rolle's theorem" after he had told them he was traveling to a math conference. Apparently he answered "it's something to do with the mean value theorem, right?" and that was good enough.