A Lindenmayer system (L-system) starts with an axiom — a short string of symbols — and rewrites it repeatedly using production rules. After a few iterations the string becomes long and complex; interpreted as turtle-graphics commands it draws self-similar fractals like the Koch snowflake, the dragon curve, or branching plants. Edit the rules, change the angle, crank up the iterations, and watch nature's recursion at work.
F/G forward (draw) · f forward (skip) · + turn left · - turn right · [ push · ] pop · | flip 180° · any other letter is a variable.