A handful of sand on a vibrating table is like a crowd on a dance floor that still holds the rhythm of the old song for a second after the DJ has switched to a new one. Scientists made a thin layer of grains bounce on a vertically shaking platform and abruptly changed the shaking intensity. Right after the jump, the granular temperature, a measure of the random motion of the grains, behaved differently than it should have under the new intensity. It remembered the old setting.

This effect is called Kovacs memory: a system that has undergone two temperature jumps, on its way to equilibrium, passes through a point where its state matches equilibrium, but then 'changes its mind' and returns. In the granular layer, researchers saw only a brief flash of such memory — literally in the first moments after the vibration change. After that, the system became amnesic: its behavior was determined only by the current temperature, as if the past had never happened.

The authors built a kinetic theory that splits the process into two stages: a fast one, where memory still exists, and a slow one, where it is gone. The first corresponds to the so-called kinetic regime, the second to the hydrodynamic regime. Computer simulations using molecular dynamics confirmed both the experiment and the theory. The main conclusion: in such a system, the full Kovacs effect is impossible due to the coupling between horizontal and vertical temperatures — memory fades before the system could 'remember' everything at once.

A thin layer of sand is a simple-looking system, but it teaches us how materials with memory lose it under non-equilibrium conditions. This matters not only for granular physics, but also for understanding glasses, foams, and other media where the past influences the present.