Rule 90: each cell's next state = left neighbor XOR right neighbor. The rule table has 8 entries (for patterns 111 down to 000); rule 90 = 01011010 in binary. Starting from a single cell, the space-time diagram is the Sierpinski triangle — a fractal with dimension log₂3 ≈ 1.585. Toggle rule bits or try other rules (30, 110, 184) to see different emergent behaviors.