Facts don't change, right? The Mississippi River still flows down the middle of the country and the height of Mount Everest is always the same.
Or is it? Turns out that many facts do change. The "facts" of the weather change all the time, of course. Even the height of Mount Everest has changed, thanks to more accurate measuring devices.
But people don't always adjust to some changes of fact, especially changes we don't really notice, and therein lies the problem.
This is the thesis of an interesting Boston Globe article by Samuel Arbesman, who argues for a category of fact called the mesofact, which describes factual knowledge that changes slowly over time.
Check out Arbesman's essay here.