The shrinkage in solid-state sintering arises from the rigid body motion of particles driven by the sintering force. Relative velocity is inversely proportional to the square of the contact area. The initial stage is characterized by the formation of contacts between particles and the growth of the contact radius. In the intermediate stage, the pinch-off of pore channels occurs consecutively, leading to the formation of closed pores, which subsequently shrink and disappear in the final stage. The fundamental sintering process at the particle scale can be understood by analyzing a two-particle model (bond formation and neck growth), a three-particle model (pinch-off of pore channels), and a four-particle model (formation, shrinkage, and disappearance of a closed pore).