Before I can begin answering it, we must first start by asking what thermal energy is. All substances are made up of tiny particles which can hold energy. The particles will move faster when they gain thermal energy in the form of heat. The change in thermal energy of a substance is represented as a change in its temperature. Hot substances are made up of particles with high thermal energy. As heat is removed from the substance (like putting it into a refrigerator), the particles lose their thermal energy and begin to move slower. As a result, the temperature of the substance decreases.

The best way to measure the thermal energy of an object is to use a thermometer. A thermometer is a glass tube filled with a liquid (mercury) which expands or contracts depending on the temperature of the object it is in contact with. It measures the average kinetic energy (one type of thermal energy) of the molecules of a substance in degrees Fahrenheit (F) or Celsius (C). Not all the molecules have the same amount of kinetic energy because the molecules are traveling in random directions at a variety of speeds, fast and slow. This is why an average temperature is taken. What we measure as the temperature is always related to the average speed of the molecules in an object.

Today, scientists use electronic sensors connected to high-tech computers or portable machines to measure temperature. The most common type of temperature sensor is a thermistor. Thermistors (or thermo-resistors) are thermally sensitive resistors that can measure temperatures very accurately and easily. A computer or other circuit measures the resistance of the object and converts it to a temperature digitally. They are very affordable, durable, and used in a wide variety of applications that require temperature measurement and control. Large scale thermistors are used in electronic heating/air-conditioning systems to keep homes warm in the winter and cool in the summer. Smaller-sized thermistors are used to measure our body temperature when we get a fever. Other temperature sensors include RTDs (resistance temperature detectors) and thermocouples. Scientists like to use these sensors because they can find temperatures faster and with greater precision than with ordinary thermometers.

Thermal energy can be described as the amount of energy a body stored due to its being at a higher temperature than its surroundings . It is measured using something called a calorimeter. Usually, the heat released by some body at some high temperature to its surroundings is compared with the known heat that a fixed amount of water at the same high temperature would release to the surroundings. The unit of heat energy is Joule, though the older unit of calorie is also used.

Thermal energy is just the movement of molecules in an object. And as you can imagine, the more jiggling there is, the more the molecules bump into each other, and the more the molecules spread apart. Because of this, as you make things hotter, they tend into actually get bigger - though some materials get bigger than others.

So the simplest way to measure thermal energy, is to take a glass tube and fill it up with some kind of liquid. As the tube heats up, the liquid heats up and expands and rises in the tube. This lets you compare the amount of thermal energy in two different things. This is exactly how they used to make thermometers, which measure temperature, though the liquid they used inside the tubes was called mercury and is very dangerous to touch. That's what temperature is, though. When you say is is 70 degrees outside, you are telling someone how much thermal energy there is in the air.

There are other ways to measure temperature because the amount that molecules and atoms jiggle also affects other things - for example, how well metals conduct electricity. So by measuring how much electricity passes through wires made of certain materials, you can tell how hot the wires are. And if you bring the wire into contact with hot water, for example, you can tell how hot the water is.