Diffusion-limited aggregation (DLA) is a growth process of particle clusters. This Demonstration simulates the random walk of many particles in a 2D bounded square, where each particle has its own state, either "free" or "fixed", and only free particles move around randomly. When a free particle collides into a fixed particle, it becomes fixed. This rule leads to the growth of complex spatial patterns made of fixed particles, which are known to be self-similar, meaning that these patterns maintain the same statistical properties at every scale (which is not apparent in this small-scale simulation). You can stop and restart the real-time simulation using the "run simulation" checkbox.