Particles wander randomly until they touch the cluster, then stick, building a branching fractal. The shape's roughness is captured by its fractal dimension (≈ 1.71 for classic DLA). Ported from my undergrad computational-physics C/OpenGL coursework.
New particles almost always hit an outer tip first. The branches shadow the bays behind them, so the inside is starved.
The screening effectZoom into any branch and it looks like a smaller copy of the whole cluster: the same pattern at every scale.
What "fractal" meansPoint, line, ring or block, sticky or not: the dimension still lands near 1.71. The big-scale shape forgets the details.
Why that matters