Hydro Cycle Water is always on the move. It falls from the sky as rain, hammers the coastline as waves, trickles through crevices 1,000 feet underground, vaporizes and sails slowly into the clouds. Even water buried under thousands of pounds of ice in the polar icecaps is on the move - its movement is slow, sometimes taking thousands of years to budge even a few inches.
Water moves continuously through a natural system called the hydrologic cycle.
Powered by the heat of the sun, all the water that falls to the ground as precipitation sooner or later makes its way back into the clouds where it eventually becomes precipitation again. In the case of water, that is certainly true. Because all of the planet's water circulates through the hydrologic cycle, Earth is a closed system. In effect, the same water is here as was here when Earth was formed. In this activity, your students will build themselves a model of the hydrologic cycle out of two-liter soda bottles and see for themselves the process Earth uses to recycle its water supply.