What do these four cities have in common?
Hints, going from west to east:
Bonus puzzle for geography hotshots
What are THESE four cities that belong in the same group as the first four (albeit not quite as cleanly, due to language variation)?
The bonus cities are Tripoli, Istanbul (not Constantinople), Sevastopol, and Persepolis. These all share the -polis root despite the variety of spellings. Istanbul literally just means "to the city". You know how people all over the Bay Area call San Francisco "the city"? Well, people near Constantinople called Constantinople "the city", and that's how it got the name Istanbul.
Solution
All four cities end in -polis (Greek for "city"). From left to right, they are Minneapolis, Indianapolis, Annapolis, and Metropolis.
Congratulations to yesterday's solvers Jacob C.🌇, Charlie🌇 (who also suggested Heliopolis, Egypt), Yana🌇, Anna J., Connor, Atticus, Mr. Gregg, Zachary S., Kate, and Graham. (The 🌇 means full points on the bonus puzzle too.) Thanks to everybody who made a guess!
Ten years later, Dr. Shapiro revived Puzzle of the Day at Proof School, writing each day's puzzle on a name tag. After 600 puzzles or so, he was just starting to feel normal about students reading his chest all the time when campus closed and the puzzle, like the rest of our lives, moved online. New puzzles are posted daily on school days.
Want to catch up on old PotDs? There's an archive currently containing puzzles from March to June 2020.