Solid or liquid fuels (chemical energy) are tangible (a tree trunk, a lump of coal, a canister of gasoline), and their burning—be it in forest fires, in Paleolithic caves, in locomotives to produce steam, or in motor vehicles—releases heat (thermal energy). Falling and running waters are ubiquitous displays of gravitational and kinetic energy that are fairly easily converted to useful kinetic (mechanical) energy by building simple wooden waterwheels—and all it takes to convert wind’s kinetic energy into mechanical energy for grinding grain or pressing oil seeds is a windmill and wooden gears
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