This light refraction phenomenon causes a distant object to appear displaced from its actual position. As such, an object can appear closer than it actually is. Because hot and cold air each have a unique density, light passes through each differently. This means that when masses of cold and hot air are layered, light will bend and create a ripple effect (similar to when waves of heat are visible on the hood of a running truck). This layered air is what displaces the appearance of an object. A common example of a mirage is the illusion of water seeming to "pool" in a dip in the highway on a hot day. Another is when the sun seems perfectly balanced on the horizon. Hills or vegetation in the distance can look close, but as one travels, they seem to get farther away. A body of water can appear in a dry desert and the distortion of light can make it seem as if people, trees, or buildings wait in the distance.