According to the second law of thermodynamics, heat is a measure of the movement or the flow of energy from hotter substances to colder ones and it is measured in Joules. In microscale, heat is known as internal energy. Two regions in thermal contact have the same temperature when there is no net exchange of internal energy between them. Heat is the net transfer of internal energy from one region to another, while temperature, which is the degree of hotness or coldness of an object, describes the average kinetic energy of molecules within substances. The faster the particles are moving, the higher their kinetic energy. Since the motion of the particles within an object is random, they do not move at the same speed and in the same direction, some of them move faster. Therefore, those particles have more kinetic energy than the others. Thermodynamic temperature can be defined for substances at (even Local) Thermodynamic Equilibrium (i.e. in condition of density/pressure which allows an efficient equipartition of kinetic energy among molecules). Temperature is then the measure of the average kinetic energy of such a system, and is usually expressed in Celsius (°C). When, particular conditions of very low pressure/density (like in the Earth's thermosphere) cannot guarantee energy equipartition among molecules (i.e. outside thermodynamic equilibrium) the concept of Kinetic Temperature should be used instead. The Celsius temperature scale is defined by international agreement in terms of two fixed points: the temperature of the ice point, which is defined as 0° Celsius, and the steam point as 100° Celsius. The Fahrenheit (°F) temperature scale is mainly used in the United States; on this scale, water freezes at 32 degrees Fahrenheit, and the temperature of boiling water is 212 F. The Kelvin scale (K) is the base unit of temperature in the International System of Units (SI). This temperature scale is obtained by shifting the Celsius scale by −273.15°; zero Kelvin is also called absolute zero.