I don't get it
swirld is a simulation of swirling fluids. Tap to add more fluid to the simulation (which changes the color), drag to swirl it around. Sit and watch it evolve. Shake for more options.
How does it work?
The Navier Stokes equation describes the motion of fluid substances. Besides being one of the most useful set of equations in physics (and it part of a $1,000,000 open math problem challenge). Unfortunately, it can be computationally quite complex, so swirld cheats. A lot (no boundary conditions, no diffusion, no true "incompressibility", etc...). This isn't designed to be an accurate physics simulation - rather a fun and colorful toy.
How do I change the colors of stuff?
Shake your iPhone/iPod Touch and the gradient picker shows up, allowing you to change the colors (you can also change the overall speed of the simulation and reset or randomize the whole thing).
What exactly is going on when I pinch/twist with two fingers?
swirld doesn't actually model colors - it models a single "density" value which it then converts to a color using a gradient. By pinching, you scale that value before it gets converted into a color (sort of like adding more resolution to a topographic map). By twisting, you adjust the "zero point", cycling the result through the gradient as you twist.