An object is said to be hot if its temperature is more than our body temperature. If we touch such an object, heat flows from it to our body and we feel its sensation. Similarly, an object is said to be cold if its temperature is lower than our body temperature. When we touch it, heat flows from our body into that object. Do you know why it happens?
We know that our body is made up of cells. And consequently these cells are made up of molecules. At normal body temperature these molecules are in motion. When a hot substance touches a part of our body, the fast moving molecules of that substance accelerate the motion of the molecules of the cells of the affected part of our body. And when the molecules of cells of that part stand the fast vibrations, the cells start breaking up. It is the breaking up of the cells that give the burning sensation. To repair the damaged cells, blood circulation becomes faster in the affected area. That is why the affected part becomes red.
When the temperature of the object touching the body is very high, a large number of cells and nerves break up. The heat of the substance dehydrates the cells of the skin and they break up. This is called burning.
Sometimes very hot substances destroy fat and bones. This causes deeper wounds in the body. The scars caused by such burning are removed by grafting the skin taken from other parts of the body.
Burns are classified into four degrees. In first degree burns only the superficial layers become red. In second degree burns deeper layers are damaged and blisters are formed. In third degree burns all the layers of the skin are destroyed. In fourth degree burns not only skin but tissues beneath the skin are also damaged.
Burns are not only caused by heat but also by chemicals, acids, alkalies and X-rays and radioactive rays.