$3.99 from gandreas software
It’s your own little world to play with in this cellular automaton based particle simulator. Set up water, fire, salt, plants, oil, soil, or viruses (or their “generators”) and watch the bits fall, rise, grow, burn, consume, float, dissolve, erode and interact with each other. And if things get stagnant, just give your iPhone a twist and watch them fall the other way. You can save multiple “worlds” (for when you’ve got something just right). Actively tweak with your creation, or just watch the element ebb and flow, it’s a relaxing way to spend five minutes or two hours.