It's typically called the Food Caravan Trick (FCT). When you deliver a food caravan to a city, it fills the food box to half full. If you deliver another food caravan to the same city on the same turn, it fully fills the food box. The next turn the city will add another person. Keep doing this and you can pump your cities up to huge size very quickly.
Would it be correct to presume that expanding
a city past 36 is only for padding the score?
I've noticed that all citizens over 36 are entertainers, cannot be changed to anything else,
and do not affect happiness. I think this
phenomenon actually occurs at (used city squares + 16); i.e. if the city only works 18 of 20
squares, the useless entertainers begin at
citizen #35 instead of #37.