...cells and muscle cells that are packed with huge numbers of mitochondria - the tiny organelles that turn food into usable energy. One way hummers can save energy is by reducing energy expenditure as the ambient temperature drops. A rufous hummer's...

...With the chlorophyll gone, the leaf not only loses its component for photosynthesis, the conversion of light into usable energy, but its healthy green coloring. When the insects leave in late June, the dying leaf is rife with colorless track...

...surface, forming hot springs. Some of it stays deep underground. The heat from that water can be converted into usable energy: That's geothermal. The U.S. Geological Service estimates that in 30 years, Alaska will have 677 megawatts...