Water is a somewhat unique substance, and is necessary for life as we know it. It has some special properties, in that it exists in three different states (as a gas, as a liquid, and as a solid) at temperatures commonly found on Earth. Water in a vapor or liquid state is a fluid, a substance that has no shape of its own. Interestingly, air is also a fluid. When two fluids are interfaced (in contact with each other), they tend to exchange properties. Water is more dense than air, and thus in most instances air tends to take on the properties of the water. The exchange of properties between air and water, and the extent to which water is a component of the atmosphere, has a huge influence on both weather and climate. The temperature of both air and water are influenced by incoming radiation from the sun (called insolation). Generally speaking, air absorbs and releases heat from insolation more rapidly than does water. Perhaps more importantly, water acts as a heat sink, absorbing insolation and retaining the heat generated by it for much longer periods of time than does air. Most of the moisture in the Earth's atmosphere got there by evaporation from the oceans.