A temperature is a comparative objective measure of hot and cold. It is measured, typically by a thermometer, through the bulk behavior of a thermometric material, detection of heat radiation, or by particle velocity or kinetic energy. It may be calibrated in any of various temperature scales, Celsius, Fahrenheit, Kelvin, etc. Measurements with a small thermometer, or by detection of heat radiation, can show that the temperature of a body of material can vary from time to time and from place to place within it. If changes happen too fast, or with too small a spacing, within a body, it may be impossible to define its temperature. Thus the concept of temperature in general has an empirical content. When conditions are such that temperatures are defined, they govern the direction of heat transfer between bodies. Within a body that exchanges no energy or matter with its surroundings, temperature tends to become spatially uniform as time passes. When a path permeable only to heat is open between two bodies, energy always transfers spontaneously as heat from a hotter body to a colder one. The transfer rate depends on the nature of the path. If they are connected by a path permeable only to heat, and no heat flows between them, then the two bodies are equally hot. If changes are slow and spatially smooth enough to allow consistent comparisons of their hotness with other bodies that are respectively in their own states of internal thermodynamic equilibrium, they obey the Zeroth law of thermodynamics and then they have well defined and equal temperatures. Then thermodynamics provides a fundamental physical definition of temperature, on an absolute scale, relying on the second law of thermodynamics. The kinetic theory offers a valuable but limited account of the behavior of the materials of macroscopic systems. It indicates the absolute temperature as proportional to the average kinetic energy of the random microscopic motions of their constituent microscopic particles such as electrons, atoms, and molecules. The coldest theoretical temperature is called absolute zero. It can be approached but not reached in any actual physical system. It is denoted by 0 K on the Kelvin scale, −273.15 °C on the Celsius scale, and −459.67 °F on the Fahrenheit scale. In matter at absolute zero, the motions of microscopic constituents are minimal. Temperature is important in all fields of natural science, including physics, geology, chemistry, atmospheric sciences and biology.