Get 'used' to it!
A while back I was on a trip in New England in the summer and went swimming in a lake. It was quite cold as I entered the lake, but later it didn't feel cold anymore. Why was that? The lake was still the same temperature, yet it wasn't cold anymore. We often refer to this as being 'used to' the water. I started thinking about it, and about heat.
Now it is sometime later and I am standing in my room in Connecticut in January looking out the window and again I am thinking about it. What is this thing that we call 'Heat'? I hold my hand against the glass in my window, thinking about what this feeling of coldness is. I imagine at the surface of my skin the dead and compacted outer layer of cells, cells that used to make up the healthy inner portion of my skin. Now the old dead carcasses and scaffolds of cells are compacted like sandbags and lay at the outermost portions of my skin. A twisted surreal mazelike canyon made of stacked skin cells that is my unique fingerprint has its highest peaks flattened against the pane of glass. I realize with closer inspection in my visualization that the pane of glass is not a flat plane at all, it is quite bumpy and rough, partially translucent. As I zoom in closer in my mind to the surface of the glass I start to perceive some strange movement, just a hint of it at first, like watching a crowd at a sporting event from a great distance. As I peer closer and closer, I see more movement, a sort of teaming about. I am imagining now seeing this pane of glass at near the atomic level. In my mind, I can make out individual molecules; they look like a sort of fuzzy and hazy globule, with no clear edge. But they are jostling about like crazy, this way and that way. There seems to be no order to the jostling, they are not jostling in uniform like a marching band stepping back and forth. The molecules themselves are heaped about in no particular order; there is no structure to them. Glass is actually what we call an anamorphic solid; it is, in effect, a liquid that has been frozen in place. Most solids are comprised of a rigid, orderly crystalline structure. When liquids like water freeze, the haphazard arrangement of molecules spontaneously locks into a specific orderly pattern. A process I can see in action as I watch ice crystals form and sweep rapidly over the surface of my windshield on a cold morning. Most glasses are not comprised of rigid crystalline structures, but are instead made of that same haphazard arrangement of molecules as a liquid that just does not flow, but the molecules never snap into an ordered arrangement. After this closer inspection of the surface of the glass, I realize that it is uneven and certainly not level, huge mounds jut out here and there like a vast land of rolling hills. Hills that are gently climbing and sloping in different directions. The hills themselves are comprised of those same teaming and jostling molecules. This movement, this teaming, is what we describe as 'Heat'. It is, in fact, just the motion of the molecules. It is actually the energy contained in the motion of the molecules, which in turn is actually just kinetic energy. So heat is really just a disguised form of kinetic energy!
As these molecules are jostling about every which way, I bring a large block close to the plane of glass. The block is by no means smooth, it is comprised of many hills and valleys and even some vast canyons. The block contains the same kind of motion in its molecules as the glass pane did, except when I look at it, I see that it has a little bit less motion. That each individual molecule is wobbling a little bit less. This block is in effect 'colder' then the pane of glass. As the block is brought closer to the surface of the glass, I can see now that the molecules are arranged in an orderly fashion, repeating over and over again along the surface. The hills and vallies are broken edges of this repetitive surface. Sort of like vast disarrayed land staircases. Now the large block is brought to touch the surface of the pane of glass. Its not really laying flat against, some hills are hitting hills on the glass, and some hills are hitting trenches and valleys, edges of the block are hitting slopes of hills on the glass pane. The molecules in the glass are still jiggling about wildly, as they 'touch' molecules of the block. They actually slam into them, much like a basketball thrown into another basketball would. When it does this, the molecule from the pane of glass actually loses a little bit of its energy to the molecule of the block that it hits. Now the molecule of the block is moving a bit quicker, and the molecule from the pane of glass a bit slower. Now that the molecule in the pain of glass is moving slower then its neighbors that it is wiggling around and bumping into, it takes a little bit of energy from its neighbors. It speeds up a bit, and its neighbors slow down. Now the neighbors are moving slower then their neighbors, and they take energy from them, and so on and so on all the way across the pane of class. Every where that a molecule of the block touches a molecule of the glass, the glass loses a little bit of energy, and the block gains a little bit. Now that the molecule at the edge of the block that was touching the glass molecules is moving a bit faster, it has more energy then its neighbors inside the block, and starts to lose that extra energy to them by wiggling about and slamming into them. In this way, the motion of the glass molecules starts to get transferred out throughout the motion of the block. After some amount of time, the amount of motion in both the glass pane and the block is about the same, they have now stabilized into an equalized temperature. The block is now a bit warmer then it was before, and the glass pane a bit colder. What has happened here is that the 'heat' from the glass pane was partially transferred to the block that it was touching.
Since a faster molecule hitting a slower one ends up transferring some of its energy to the slower one, and losing some of its own, heat always 'travels' from a hotter material, that is a material that contains more kinetic energy, to a cooler material. Now I picture my finger again, the surface of which is packed with sandbag layers of dead cells. The surface of those cells as I zoom in are made of numerous small bumps. These bumps, the atoms the cell is made of, are individually jolting about, moving a little bit this way and hitting its neighbor and deflecting back that way. The motion of all these atoms is the heat contained within me. As my finger moves closer to the surface of the pane of glass, some of the dead outer cells start coming into contact with the surface of the glass. The glass molecules are jostling about less then the atoms and molecules that make up my outer skin cells on my body. The outer skin cells now start to lose their excess of energy to the molecules of glass. Those outer skin cells now have less kinetic energy among themselves than the atoms and molecules a little further in, and they start taking energy from them, transferring it to the glass pane. As this cascading transfer of heat from more energetic molecules to less energetic ones penetrates like a storm front deeper into my finger, it starts taking heat from living cells. Now my outermost layer of living cells contain less energy then the ones farther in, and start taking energy from them as well. As I move further in from the outer layer of dead skin cells deeper into my fingertip, the amount of energy increases. At the points where my outer dead skin cell molecules are in contact with the molecules of the glass, their temperatures are now about equal. As I move further into my layers of finger cells, the temperature gets closer and closer to my normal internal body temperature. I now realize that at some point the outermost layer of living skin cells contains less energy then the inner most layer, they are in effect 'colder'. The living skin cells have an intricate mechanical structure that inherits the kinetic energy of the cell it is contained in. These mechanical structures within the cells are jostling more in the cells that are further inside the finger then the outer layer of living skin cells mechanical structures. A small nerve connects these structures in a complex manner to my brain. When my brain gets the signal that the outer layer of living skin cells are jostling about at this temperature, and the innermost layer of living skin cells is jostling about at that temperature, the brain reports that difference as a feeling of coldness. The more the difference between those inner and outer living cells the more 'cold' something feels, conversely, the less the difference between those the less cold they feel. So after an initial contact with a very cold surface, a great amount of difference is reported to the brain, and a strong perception of cold is felt. But after a while, the outer layer of dead skin cells is the same temperature as the surface of the cold material, and the inner layer of dead skin cells is as well. Soon after that, the outer layer of living skin cells are the same temperature as the cold surface, and soon after that the inner layer of living cells are the same temperature as the cold surface. Now all the outer layers of my skin are the same temperatures as the cold surface they were in contact with. My brain now reports that the energy difference between those varying layers of living skin cells is minimal, it no longer reports a 'cold' sensation. Hence, I get 'used to' a cold lake after I have been in it for a few minutes! The outer few layers of skin cells are now about the same temperature of the lake, and no longer report that difference in temperature. Even though the lake is just as cold as it was before, and I am almost as warm as I used to be, I now no longer feel the coldness of the lake.
So as I hold my finger against the plane of glass, my brain reports a sensation of coldness. As I let it sit there the cold front, that is, the wave of decreasing energy, is penetrating deeper into my finger, and after it passes the heat sensitive layer of skin cells, I no longer feel the coldness. If some mechanism were in place that heated me internally, I would not sense it, for the body has much more limited heat-detecting capabilities inside. This is because most time I would need to know if I were being heated in those critical, evolutionary-pressure driven generations I could have only been heated from an outside source, the sun. There weren't too many naturally occurring microwave ovens or radio frequency generators.