Representative time evolution of the central cooling state of a massive galaxy cluster. In particular, the central entropy of the most massive z = 0 halo in TNG-Cluster is given by the circular symbols (color indicating relaxedness, in terms of the ratio of kinetic to thermal energy within r_500c). Correspondingly, the light gray line shows the smoothed time evolution of K₀, while the dark gray curve shows its time derivative. Local extrema of the derivative are indicated by crosses, with larger orange crosses marking significant core transformation events (see text). The blue markers indicate the timescales or duration of the two core transformations (see text for a definition). This halo undergoes a core transformation between z ∼ 0.3 − 0.5 wherein the core evolves toward a more CC state. In this case the transformation is temporary and lasts ∼2 − 3 Gyr, after which the central entropy returns to its previous level. This type of behavior is one of several archetypes (see text for discussion). The pink stars mark the start of the three most massive merger events in the history of this cluster. The size of the markers indicate the mass ratio of the merging event. We mark the formation time of the cluster with the vertical dashed silver line. Note that throughout the paper, time evolves from right to left in all plots that depict a time evolution.