The hydrological cycle refers to the stages water takes as it circulates through the land, oceans, and sky. The Sun's heat causes water from lakes, rivers, and oceans to evaporate into the atmosphere, where it eventually condenses into droplets in clouds. When the clouds meet cool air, precipitation occurs in the form of rain, sleet or snow. This phenomenon helps to regulate the earth's climate, making it conducive to life. This condensation and rarefaction suggested to early thinkers how a single element might become others, or how the several elements might be related. Aristotle called it the "lesser circulation," the greater being that of the planets and stars. William Harvey drew on this deep metaphor in his discovery and proof of the circulation of the blood, the lesser being pulmonary and the greater being throughout the body. Modern physics' investigations into what are now called phase changes was essential to the development of entirely new theories of heat and cold, leading to thermodynamics and theories of something called "energy," that might change its shape and nature and thereby power change. So-called latent heat that is stored or released in such phase changes is behind modern refrigeration and air conditioning.