Water

The uneven distribution of charge in water molecules allows them to bond to each other and to surfaces, enabling them to move against gravity, including up plants through roots and stems. These bonds also keep water in a liquid state over a wider temperature range and make it less dense as it freezes. This causes ice to float and form an insulating layer for underwater ecosystems, from which all life originated.

Electricity can be modeled as the transfer of energy via the flow of charged particles. Although water molecules have one negatively charged and two positively charged ends, they are neutral overall and cannot carry an electric current. However, water can easily dissolve many other chemicals, releasing charged ions that enable impure water to conduct electricity.

About 97% of Earth's water volume is in the oceans, 2% is trapped in polar ice caps and glaciers, and almost all of the remaining 1% is found in groundwater reserves. Roughly 70% of the easily accessible surface water in rivers and lakes—0.01% of all water—is used for agriculture, contributing to 20% of the world's population living in water-scarce areas and 2.1 billion people lacking access to clean drinking water.

Because water is lost through breathing, sweating, urination, and bowel movements, consuming 2.0 to 3.7 liters (about 4.2 to 7.8 pint glasses) of water daily is recommended, depending on various factors, such as health conditions and exercise habits. Fruits and vegetables that are almost entirely water by weight, such as strawberries, cucumbers, and broccoli, can help increase water intake.

When we're born, about three-quarters of our bodies are composed of water, dropping to an average of 55-60% for the average adult. The lungs are over 80% water, the kidneys and spleen about 80%, and the heart and brain are approximately three-quarters composed of water—about the same moisture as a banana.

As water is heated, more of its molecules acquire enough energy to overcome the surrounding atmospheric pressure, allowing them to escape the liquid surface and evaporate into a gas. Because air is thinner at higher elevations, atmospheric pressure is lower, meaning water can surpass the threshold atmospheric pressure with less energy and thus lower temperatures.

Despite covering almost three-quarters of its surface, water only makes up about 0.12% of Earth's volume. Water in some of Jupiter's and Saturn's moons comprises a larger share of those celestial bodies and of the total water in the solar system. For example, Jupiter's moon Ganymede is believed to harbor almost nine times Earth's oceans beneath its icy shell.

While a single drop of water spends an average of nine days in the atmosphere before precipitating back to Earth, it can spend thousands of years in the ocean before evaporating, and hundreds of thousands of years deep in rock layers before being released as steam in volcanic eruptions. Some water molecules have been trapped in Antarctic ice sheets for millions of years since they last moved through the water cycle.

In 2010, the UN General Assembly recognized access to water and sanitation as a basic human right, noting that an estimated 884 million people worldwide lack access to safe drinking water. The treatment of water as a commodity and reserves as property has contributed to unequal access and competition over its use, which is expected to intensify due to climate change.

This interactive gallery highlights the various types of water behind the production of biofuels, foods, and materials such as cotton. Diets high in bovine meat, whose production requires 15,415 liters of water per kilogram (1,847 gallons per pound), are among the most water-intensive, whereas every kilogram of vegetables uses 322 liters of water (39 gal/lb) to produce.