In statistical mechanics, the Kac ring is a toy model[1] introduced by Mark Kac in 1956[2][3] to explain how the second law of thermodynamics emerges from time-symmetric interactions between molecules (see reversibility paradox). Although artificial,[4] the model is notable as a mathematically transparent example of coarse-graining[5] and is used as a didactic tool[6] in non-equilibrium thermodynamics.