Diving Beet is a port of Falling Turnip from Haskell to Futhark and Python (or Go; see below). It is a physics imitation that uses cellular automata to implement fascimiles of gravity and (al)chemical interactions between various elements. For example, water plus fire gives steam, lava melts sand and metal (the latter at a slower rate), plants grow into water, etc.
You can look at a video of it in action - note that the choppiness (especially near the end) is due to the screen recorder not being able to keep up (it runs out of memory and starts writing to disk).
Just make run, although you'll need the Futhark compiler installed.
You can insert particles by left-clicking, although you can't
overwrite anything that's already there. Right-click to delete
particles. Use the left/right arrow keys to select the particle to
insert, and the scroll wheel to control the modification radius.
Diving Beet has also been implemented with a GUI written in Go. This
performs much better on some systems. Make sure you have Go and the
packages github.com/veandco/go-sdl2/sdl and
github.com/veandco/go-sdl2/ttf installed, and then run:
$ make diving-beet
$ ./diving-beet