Vaguely inspired by some numerical simulations that recently populated the web, I will talk about collisional clusters of interacting particles, focusing on elementary hard sphere dynamics. Different notions of clusters may be typically bounded, or not, depending on their behaviour under time-reversal. Cluster dynamics is a tendency of self-organising in small groups (clusters), which behave independently for random intervals of time. This breaks with the sudden appearance of a giant cluster. Based on a kinetic limit, one can provide analytic formulations.