How does some kinetic energy become internal energy during an inelastic collision?

In an inelastic collision, some of the external kinetic energy of organized motion is irretrievably transformed into internal energy.1

From an atomic perspective, the total energy of a body consists of external energy—associated with its position, center-of-mass motion, and spin—and internal energy, which is determined by the arrangement and motion of its constituent atoms. During an inelastic collision, while the sum of internal and external energy remains conserved, a portion of the kinetic energy is converted into random internal motions of the atoms and molecules that constitute the bodies.12

In macroscopic systems, this transformation typically manifests as heat, which raises the temperature of the colliding objects. This process is an example of the irreversible degradation of organized motion into random internal energy, a phenomenon described by the second law of thermodynamics. In microscopic collisions, such as those between atoms, the lost kinetic energy may instead excite the particles into states of higher internal energy.12