Energy
A physical quantity expressing the capacity to do work, existing in forms such as kinetic, thermal, electrical, and chemical energy that convert into one another. In an isolated system its total amount is conserved.
Each night, a Hummingbird enters torpor: its heart slows from up to 1,200 bpm to under 50, body temperature drops from 40°C to ~10°C, and metabolism falls 95%. Without this near-death energy saving it could starve before dawn; wing-muscle tremors wake it before sunrise.
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The Kardashev Scale measures a civilization's advancement by energy usage. Type I harnesses all energy reaching its planet; Type II, all from its star; Type III, all from its galaxy. Humanity currently sits at about 0.7 on this scale.
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