The object of this quiz is to click on the circle containing the largest population. When you begin you'll be shown 2 circles on the map with radii between 10 and 250 miles which do not intersect. Click on the one with the larger population within it (based on US census tract info) then 'Submit' - if you are correct you'll move on. One incorrect round and the quiz is over. Every 5 rounds the number of circles on the map will increase by 1. Try to build the longest streak of correct circles. Circles displayed will always be non-intersecting.
NOTE: You may have to zoom out to see all circles, occasionally a AK/HI circle will be included. Also, only population within the US is included in the total, any Canada/Mexico coverage counts as 0 population within the circle.
Quiz Takers 
Based on how your quizzes define “cities,” shouldn’t this list also include towns in New England at state tri-points (and one in far northern New Hampshire that spans between Vermont and Maine)?
I think you might also be missing two at the tri-point of New Jersey, Delaware, and Pennsylvania, and two or three (hard to tell from Google Maps) at the tri-point of Kentucky, West Virginia, and Ohio. Some of these places are pretty obscure, but it seems they should count all the same.
I also don’t see the city limits of Joplin touching either state boundary in any maps I’ve looked at.
Which places, you can name them here. Joplin does in fact have a tiny part that reaches the KS-OK tripoint, if you Google Joplin city limits you’ll see a couple maps.
what about cairo, il?
Doesn’t border, bridges don’t count.
?? This doesn’t have anything to do with bridges. The city limits of Cairo include portions of the border between IL-KY and IL-MO. Those borders are in the Ohio and Mississippi rivers, resp. But it’s no different from Weirton, which is across the Ohio river from OH, or East Liverpool which is across the Ohio river from WV.
Or Sioux City which is bordered by the Missouri (NE) and Big Sioux River (SD). Or Washington, DC. which is across the Potomoc from VA.
Ah I see, for some reason wasn’t in the census listing of such cities – will add.
Philadelphia doesn’t count?
nope city limits do not go down to DE