some puzzle that can be found online are typically a 3x3 grid where you've got nine spots filled with some images or symbols or geometric figures and you have to predict what the missing one should be. Some of them are made out of symbols, some other are little 3x3 grids (inside the big grid) with some number of symbols (crosses, asterisks, etc..) placed according to some pattern and, again, you have to guess what pattern the missing grid would be.
Does anyone have any reference where I can find how they can be solved, or any theory behind this kind of puzzle?
Again, I am not looking to the specific solution of the examples above. I'd like to have a theory behind it, or tips to solve them.