Natural convection is the state of temperature dictating the position of more- or less-dense substances. The classic example is water; heated water is less dense than cool water, and so it rises while the cooler water sinks: this is a property of buoyancy. One of the physical properties of convection is that it requires density to resist against an outside force to occur; for example, most convection on Earth, including the convection of hotter and cooler air, occurs because the fluid density is moving in response to the Earth's gravitational field. This means that in a weightless environment, such as near-Earth orbit or out in intersolar space, natural convection could not occur. Without the buoyancy properties -- weight as opposed to mass -- the fluid cannot react to changes in density by rising or sinking; instead, it remains in place, and must be moved by an outside force such as a fan or pump. This means, among other things, that air-moving fans are vital to human life in outer space, as the exhaled carbon dioxide will not rise but instead remain in place, and cause the air quality to become more toxic.