What creates heat?

Matter is made up of atoms and molecules (groupings of atoms) and these atoms and molecules are constantly jiggling around, bumping into each other. When the molecules (for example a water molecule in the air) bump into our skin, there is a transfer of energy. If they hit our skin with less force than the molecules making up our skin already are hitting each other with, our skin-molecules will go a bit slower, and the other molecules will go a bit faster after the collision, and we will feel colder. If they hit with more force than the skin-molecules had, the skin-molecules will go faster and we will feel warmer. The energy that speeds the atoms and molecules up in the first place comes from many, many different sources. For example when you burn wood, the chemical reaction between the wood and the oxygen in the air to produce ash, CO2 and water spits out the new molecules with a pretty decent force, and it is therefore not smart to hold a burning log in your hand. They get such great speed that they begin to glow, what we see as a flame (a phenomenon that has to do with electromagnetism). When this light hits your body, the molecules in your skin absorb it and starts to jiggle faster, which is why you can feel the heat from a fire even when you are not touching any of the fast molecules from the reaction. This is the same reason why you want to sit in the shade on a hot summer day. You want to absorb as little of the light from the sun as possible. The thing that makes the sun so incredibly hot in the first place is another way to get heat: fusion. Here several light atoms combine to create a heavier atom, and afterwards spit out a particle with insane speed, enough to heat the interior of the sun to millions of degrees. In the end almost every physical process creates some amount of heat. Rubbing your hands together takes chemical energy stored in your muscles and turns it into heat, both in the muscles themselves and in the skin of your hands. Stopping a car takes the kinetic-, movement-energy of the car, and turns it into heat in the breaks. Reading this sentence takes energy stored in chemicals delivered to your nerve-cells by your blood and turns it first into heat and electrical impulses, which are your thoughts, before they too ultimately become heat due to the electrical resistance in your brain.