What rule does border expansion follow?

Does anyone have any insight into what rule the cultural border expansion follows?

Weakest tiles first?
Strongest tiles first?
Random?

Solution 1:

You can see which tile your city is going to go for next by opening the city screen and looking for the hex with a purple border. It definitely isn't random, it tries to pick the "best" tiles. Generally it seems to go for resources first, but it also doesn't seem to go for tiles that are 3 hexes away until it has gotten all the ones that are two hexes away. Of course, you can always purchase a tile yourself if it is one you really want.

Solution 2:

It acquires tiles in order of "shortest distance" in terms of how many movement points it would take to get to that tile from the city. So a flat tile two (or even three) squares away would be acquired before a hill/forest two squares away. I don't believe that it uses the tile value, but it may favor resources against resourceless at the same distance.

You can see which tile it's going to acquire on the city screen as it will be colored magenta on the city tile map.