The most elementary and "most human" measuring method is "counting on the 5 fingers". Phenomena that have the property of countability are denoted to be quantized. It begins with the composition of the matter out of small components like atoms, ions or electrons and passes through the science up to complex questions also of the thermodynamics.

More than a hundred years ago, Max Planck discovered that energy cannot be found in any size, but only in very small portions. Such an energy portion has since been called an energy quantum. (Plural: energy quanta) In all types of energy and energy changes, the question always arises as to how a substance or a material system reacts to mechanical or thermal work which the system executes at the environment or which the environment performs at the System. Substances in isolated systems have a memory for these storage processes:

If work was performed on them, they are able to execute a same amount of work again on another system.

However, a poor insulation also causes a kind of memory loss, so the system can then do less work. In order to understand these storage processes, it is reasonable to use such objects as a model for this storage that are familiar to us from everyday life and also serve to store something, whether it be a shelf or a cylindrical vessel.
Without elaborate mathematics, the shelf model as well as the fluid model allow this understanding of thermodynamic basic phenomena of entropy and thermocapacity on the one hand and their relationship to temperature, thermal energy and thermal radiation on the other hand. Low-cost shelves which we often find in take-away markets of furniture show many analogies between the storage of everyday items on shelves and the storage of thermal energy by chemical substances.
The fluid model uses the physical properties of a fluid medium to simulate processes of the storage of thermal energy and thus to make them understandable. Similar to the shelf model it describes mechanical analogies to temperature, thermal energy, thermal radiation, entropy and thermal capacity. It is able to visualize quantum thermodynamically calculated values of the stated physical quantities very differentiated. In its simplest form it can already be used in school lessons, but it allows several refinements even to the university level.

In today's didactics, secondary school classes hear for the first time something about the energy quantization when the lesson deals with the structure in the atomic shell of electrons. Since electrons are particles with fermion properties, this topic does not appear suitable as an entry. It would be much easier to introduce the quantization earlier in the case of the chemical particles because there are less secondary conditions for these particles. Thus the elementary thermodynamic processes, such as heating, cooling, mixing, chemical reaction or chemical equilibrium, can be explained as quantum phenomena.

On this website we try to avoid the usage of the term 'heat' as a scientific technical term. In scientific context 'heat' has prooved to be an unfavorable term. In thermodynamic context it has been used since the 19^th century to describe a certain property of a thermal work process - but the semantical meaning of 'heat' is not that of a process. This faulty use has been producing inconsistencies up to the present time since the 19^th century. To avoid errors of comprehension we use the following terms:
Thermal energy describes a quantum energetic state.
Thermal work describes a certain process changing the energetic state of a system by
thermal radiation as emission or absorption.
Thermal capacity and entropy describe thermal properties of matter.

Further details on both models you find by clicking on "The Shelf Model" or an "The Fluid Model" in the left frame.