What is Energy?
No one really knows, but whatever it is, it is conserved. The word comes from the Greek ἐνέργεια - energeia (activity, operation) and ἐνεργός - energos (active, working). Energy is described as the ability to do work. Energy is defined as a scalar physical quantity with the SI unit of joules (J). Still not satisfied with these answers? Here is what Richard Feynman,the celebrated physics teacher and Nobel Laureate, had to say about energy during a 1961 lecture at the California Institute of Technology:
There is a fact, or if you wish, a law, governing all natural phenomena that are known to date. There is no known exception to this law—it is exact so far as we know. The law is called the conservation of energy. It states that there is a certain quantity, which we call energy, that does not change in manifold changes which nature undergoes. That is a most abstract idea, because it is a mathematical principle; it says that there is a numerical quantity which does not change when something happens. It is not a description of a mechanism, or anything concrete; it is just a strange fact that we can calculate some number and when we finish watching nature go through her tricks and calculate the number again, it is the same.
—The Feynman Lectures on Physics
Now, just to clear it all up, since 1918 we know that the law of conservation of energy is the direct mathematical consequence of the translational symmetry of the quantity conjugate to energy, namely time. That is, energy is conserved because the laws of physics do not distinguish between different moments of time (see Noether's theorem). Wow, it is simple after all!
Heat Energy Demonstrations
Here is an illustration of heat energy. It is a short movie showing the "quenching" of two different blocks of copper. The room temperature copper blocks are placed into a bowl of liquid nitrogen. One block is clean (normal) and the other block is coated with teflon grease (a very good thermal insulator compared to the copper!). Let's see what happens:
Copper blocks quenched in LN2
Another interesting illustration of heat energy involves placing one's hand into liquid nitrogen!
Hand into LN2 and the Leidenfrost effect