Pretty interesting game. This is just one of many like it. I highly suggest reading the wiki on this before you start otherwise you will have no idea what to do and just start drawing random lines like I did

The Falling Sand Games are a series of particle-based computer games which allow the user to place particles of different elements on a canvas. The particles can interact with other particles in various ways, and may be affected by gravity. Many complex effects may be achieved. For example, fire can be made to move upwards, burn elements, and emit smoke. Game physics files (mods) are generally standard ASCII text files which tell the game how to control the particles. Many versions of the Falling Sand Game have been written since its introduction in 2005.

play it here