The screen is divided into rows and columns. Atoms can spontaneously move, appear, and disappear. Neighboring atoms can join to form molecular chains. Molecules can move, rotate, and molecular bonds can break. The processes are stochastic.
Simple molecules form, move, and break apart.
With careful observation and measurement, the first few rules and probabilities could be deduced relatively quickly. Some of the later rules which govern molecular dynamics would take considerable effort to learn from observation. If you start with a blank screen and leave the program running for a few days, you find something interesting. Moreover, it has another unique property. It functions as a clock.