Conway’s Game of Life

Conway’s Game of Life can be a good introduction to using computers for modeling physical systems.
The Game of Life is a cellular automaton devised by the British mathematician John Horton Conway in 1970. It is the best-known example of a cellular automaton.
http://www.bitstorm.org/gameoflife/

http://www.math.com/students/wonders/life/life.html

http://en.wikipedia.org/wiki/Conway’s_Game_of_Life – has lots of links to other sites and apps to try it out.