Energy is described as the ability to produce changes from one system to another. This generally occurs because of an energy transfer from one system to the other. Mechanical energy is energy that has to have a medium, or source, through which to operate. Sound waves are an excellent choice here, as they will not travel without air to travel through. Chemical energy is produced by combining two elements or compounds which will react with each other. One of my favorites is takine hydrogen gas, combining it with oxygen gas, giving a small spark, and POP! A small explosion occurs, with the production of water, H2O. Thermal energy is anything taking on heat or giving off heat; when you talk about why a refrigerator works, it is because it subtracts heat from the interior of the refrigerator. Electrical energy works because of a flow of electrons from a high electrical potential to that of a lower electrical potential; when you turn on a flashlight, the electrons flow from the positive terminal of the battery through the light bulb filament, causing it to glow and give off light, and returns to the negative end of the battery. And last, but not least, light energy occurs as part of the electromagnetic spectrum; light radiates from its source as part of a specified frequency within the spectrum. Light from the sun is an excellent example here, taking 8.3 light-minutes to cross the 93 million miles from the sun to the Earth to get here.