A good sports rivalry is often defined by geography and history, cemented over the years by memorable moments, temporary bragging rights and (usually) good-natured insults between dueling fan bases.

The best rivalries stand the test of time — but a lot of them are far more fluid than we might initially think.

That line of thinking was sparked Tuesday morning by an ESPN.com piece attempting to define the biggest rival for all 32 NFL teams. When it came to the Packers, the site appeared (temporarily) confused, listing the Vikings as Green Bay's top rival in introductory text while asserting later on that it was Chicago. Later, the piece was updated to reflect only the Bears being the Packers' biggest rivals.

For the Vikings, the Packers were listed as the biggest rivals — and it's doubtful anyone who puts on purple face paint on Sundays would disagree.

The thing is, a lot of us can remember a time when the Bears were the Vikings' biggest rivals instead of the Packers.

And even the most devout Packers fans can envision a shift in their biggest rivalry. While most of those I polled informally on Twitter said it is still the Bears — and it's hard to argue with an all-time series that goes back to 1921 — a good number also added the Vikings have gained ground over the years (and specifically recently, when the Bears have managed just six winning seasons out of the past 20).

On the first point, if you are of a certain age — and some of us who will turn 40 later this year are of that age — you remember the Vikings/Bears games of the mid-1980s to early 1990s were a huge deal and the Vikings/Packers games were often afterthoughts. Mike Ditka. Stellar defenses. Dramatic finishes. That was the big rivalry.

Brett Favre's arrival in Green Bay — along with the Bears' aforementioned tumble into mediocrity — changed all of that. Packers vs. Vikings became appointment viewing on this side of the border. Randy Moss added fuel for the fire. Favre joining the Vikings at the end of his career took things to an entirely new level. And now here we are, with the Packers being the Vikings' clear No. 1 rival. We'll add another layer on Sept. 18, when the Packers are the Vikings' first opponent in U.S. Bank Stadium — in primetime, no less.

On that second point, those key moments in the Vikings/Packers rivalry over the past quarter-century have helped close the gap from the perspective of Packers fans, but the history and tradition of the Bears rivalry still keep that one in the top spot.

That said, if you add one more fierce period of competition between the Vikings and Packers — which the next five years very well could be, assuming Aaron Rodgers continues to play at an extremely high level while the Vikings demonstrate last year was the start of good things to come — it wouldn't be surprising if a lot of Packers fans had a different answer to the rivalry question in, say, 2020.