Once during a mathematical conversation with a student, Alexander Grothendieck was asked to consider an example of a prime number.

"You mean an actual prime number?" The student replied, "Yes, an actual prime."

Grothendieck then said, "Alright then, take $57$".

-Taken from the Comme Appelé du Neant article in the Notices of the AMS

Grothendieck then said, "Alright then, take $57$".