The water cycle is primarily powered by energy from the Sun. The Sun's heat causes water in oceans, lakes, rivers, and plants to evaporate or transpire into the atmosphere as water vapor. This vapor rises, cools, condenses into clouds, and eventually falls back to Earth as precipitation, continuing the cycle. Gravity also plays a role in moving water back to Earth's surface through processes like precipitation and runoff, but the main energy source driving the water cycle is solar energy.