Whenever you put your finger in a cup of hot coffee, you can feel that the temperature is warm. This is because the water molecules have a higher average kinetic energy than in a colder cup. Moreover, the molecules have more disordered microscopic motion because they are moving faster. Similarly, when a hot body comes into contact with a cold one, heat flows from the warmer body to the cooler body. This process is called thermal equilibrium.
Temperature is measured using a device called a thermometer. Different units of temperature exist, but the most common in the United States are Fahrenheit and Celsius. In some countries, the unit is Kelvin.
In addition to the temperature scale that a thermometer uses, scientists also use black-body radiation as a way to determine the temperature of objects. For example, a piece of wood will emit infrared radiation when it is hot. The intensity of this infrared radiation is proportional to its temperature. A thermal imager, which is used by NASA and the Department of Defense to remotely measure the temperatures of buildings and other large structures, is based on this concept.
There are many reasons we need to know the temperature of an object. It can prevent product damage, ensure sterilization, make sure mixtures are blending properly, control chemical reactions, and determine biological health. It is also a regulatory requirement in some cases, such as with food and drug products. We can even use temperature to predict how fast a material will expand, as is the case for glass when it is heated.