An excellent question,

the concept of temperature relates to the average speed of molecular movement. It is a relative measurement, letting us know simply that one object is hotter (or colder) than another object.

The principle of temperature measurement is essentially linked to the so-called “Zeroth Law of Thermodynamics”. This curiously named law is normally expressed in two ways.

• Two objects than can exchange energy with each other, but don’t exchange energy with each other are at thermal equilibrium.
• Two objects that are independently at thermal equilibrium with a third object, must be at equilibrium with each other.

These two statements are essentially how we measure temperature. You must first understand the concept of hot and cold. If you were to jump into a cold swimming pool the energy will be moving from you, to the pool, the movement of the energy from you to the pool means that you will perceive it as being cold. If the pool was hot, the energy would flow into your body and you would perceive it as being hot. If however the pool was the same temperature as you were, you would neither perceive it as being hot or cold. This would be a case where two objects (you and the pool) could exchange energy, but do not exchange energy. You and the pool are at thermal equilibrium. What if you wanted to know what you would feel before you jumped into the pool? Simple we use the concept of temperature. First of all you measure your own temperature, using a thermometer it tells us that it is at 37 degrees C, this will be correct so long as the thermometer is left under your tongue for long enough to reach thermal equilibrium. Then you measure the temperature of the pool, and lets say that is also 37 degrees C. Both objects (you and the pool) are then at thermal equilibrium independently with a third object (a thermometer at 37 degrees C), so you and the pool are then at equilibrium with each other. 