My october game is Magic Fog, a puzzle game built on top of a computer simulation of fluid flow. By swirling around fluid with your mouse you create "magic turbines" to power little magical gadgets that pump the fluid around and shoot at enemy bugs.
There are many types of algorithms for simulating fluid (which I will write up in a later blog entry). And I spent the first two weeks of October trying out several of them. I finally settled on the Lattice Boltzmann Method since it was easy to write and fairly easy to tweak. There were also several code samples that implemented a full 2D simulation in a few dozen lines of code, which helped a lot as well!
Once the fluid simulation worked, I went to work making some tutorials and puzzles. I discovered that making good puzzles based on flowing goop is not that easy. As a result, the three puzzle levels will appear either too hard or incredibly frustrating to a typical player. Still, I think with some tweaking of the simulation and some better gadgets (which I called "whimsies" for some reason) it would make a good basis for a puzzle game.