Asteroid and comet

Rock that stays rock, and ice that grows a tail when it gets warm.

The traditional split is composition and behaviour: asteroids are rocky or metallic and inert, comets carry volatile ices that sublimate near the Sun and produce a coma and tail. The boundary has turned out to be leaky. Some asteroids sprout tails, some comets exhaust their ices and go dormant, and 3200 Phaethon is catalogued as an asteroid while producing the Geminid meteor shower. The useful modern distinction is where an object formed rather than what it does now. Formation location is the modern line because it maps onto composition without depending on behaviour: beyond the snow line water was solid and got built in, inside it water was vapour and did not. An object that formed outside and has since been baked dormant is still a comet by that reading, which is a definition that survives the object changing.