Up and down are meaningful near a large mass, like a planet. “Down” indicates the direction toward which the large near mass exerts gravitational pull; “up” indicates the opposite direction. The same goes for “hot” and “cold”: there are no “hot” or “cold” things at the microscopic level, but when we put together a large number of microscopic constituents and describe them in terms of averages, then the notion of “heat” appears: a hot body is a body where the average speed of single constituents is raised.
