On very small scales, an object's kinetic energy tells you its temperature, and on larger scales, an object's kinetic energy tells you its velocity. Where's the cut off? I mean, velocity is relativistic, but temperature... isn't... is it? Also, black body radiation decreases a body's temperature over time, but not its velocity. So what's the difference? Related question: How big can a molecule get before its kinetic energy stops telling you how hot it is and starts telling you how fast it is?