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Heat and temperature are closely related but distinct and sometimes subtle ideas. Heat is simply transferred thermal energy—most commonly, the kinetic energy of molecules making up substance, vibrating and bouncing against each other. A substance's temperature, on the other hand, is a measure of its ability to transfer heat, rather than the amount of heat transferred. For example, a match lit under a pot of boiling water reaches a much higher temperature than the water, but it is able to give off much less heat, since only a small amount of thermal energy is created and released by it. When any two substances of different temperatures are in thermal contact, the laws of thermodynamics state that heat flows from the higher-temperature substance into the lower-temperature substance, raising the temperature of the heated body and lowering the temperature of the body releasing heat until thermal equilibrium is reached, and the temperatures are the same. Thus temperature describes a characteristic of matter that determines the direction and extent of heat transfer, so the match with little heat but high temperature still adds energy to the water when placed under the pot. Providing a closed physical system with heat generally raises its temperature but not necessarily; for example, ice at zero degrees Celsius requires considerable additional heat in order to melt into water at zero degrees Celsius. Temperature can be related to the average kinetic energy of the molecules of gases, though this relation breaks down in most real cases involving liquids, solids, substances with larger molecules, and radiation with no mass, such as light. The two most common temperature scales, Celsius (C) and Fahrenheit (F), are based on the freezing and boiling points of water. On the Celsius scale there are 100 increments between the two points, and on the Fahrenheit scale there are 180. Scientists also use the International System units called Kelvins (K). A difference in temperature of one degree is equivalent in the Celsius and Kelvin scales, but their absolute scales are different: while zero degrees C is the temperature at which water freezes (at a pressure of one atmosphere), zero degrees K (−273.72 degrees C), also called absolute zero, is the least possible temperature for a system, representing a theoretical state from which no heat can be extracted.