Ballistic DepositionFlakes of snow fall straight down onto a flat ledge. Each one sticks the instant it touches a neighbour, so it can catch on the shoulder of the column next door and leave a gap underneath. The surface never stays smooth — it roughens into ridges and valleys that keep growing.Kardar, Parisi and Zhang wrote the equation for such a growing edge in 1986. Its roughness climbs as time to the one-third power — the same exponent turns up in burning paper, bacterial colonies and coffee-ring stains, a whole “KPZ universality class.”