Water is constantly in motion. Sometimes it moves quickly, like a fast-flowing river, but sometimes it moves quite slowly, like in underground aquifers, glaciers, and deep ocean currents.
Water moves through Earth's systems in a cyclic fashion taking many forms as it travels. This process is known as the hydrologic or water cycle. The water cycle is often shown as a simple circular cycle (as in the accompanying diagram) in which water evaporates from the ocean, is carried over land, falls as rain, and then travels back to the ocean through rivers.
Although a drawing of the water cycle oversimplifies the actual movement of water, the diagram is a useful tool.
The actual path any given water molecule follows in a complete water cycle can be varied and complex and may not follow the exact path shown by a diagram.
Water cycle diagrams do not show the amount of time that a water molecule may take as it travels through the water cycle. For instance, water starting in the Antarctic may take over 250 years to travel along the bottom of the Pacific Ocean before it re-surfaces near Alaska. Water can also remain frozen in a glacier or ice sheet for thousands of years before re-melting.
Water may also change state, back and forth, from a liquid, gas, and solid (condensing, evaporating, etc.) as it travels through the cycle. Water even travels underground, where it seeps through the spaces between grains of soil, sometimes coming to the surface as artesian springs, where water will flow to the surface without pumping because of natural pressure from below.
Living organisms also move water around. Water is either directly consumed as liquid or extracted from food and then carried within bodies. It leaves the organism as a gas during respiration, is excreted, or may evaporate from the skin as perspiration. Plants are the major biotic movers of water. Their roots collect water for distribution throughout the plant. Some of the water will be used in photosynthesis, but most travels to the leaves where it is easily evaporated.

Which of the following statements is true?
• Each drop of water follows the same exact path in the water cycle, but the speed may vary.
• Each drop of water follows a unique path through the water cycle, and its speed may vary.
• Each drop of water follows the same path through the water cycle at an inconsistent speed.
• Each drop of water follows the same exact path in the water cycle at a consistent speed.