The phrase 'the fog comes on little cat feet' in Carl Sandburg's poem 'Fog' is an example of

Sandburg gives the fog the characteristic movement of a cat, attributing animal qualities to a non-living natural phenomenon, which is personification. The fog is described as moving silently and stealthily on 'little cat feet,' making it behave like a living creature. This device makes the abstract and intangible quality of fog more vivid and concrete.