An eyebrow is an area of short hairs above each eye that follows the shape of the lower margin of the brow ridges of some mammals. In humans, eyebrows serve two main functions: first, communication through facial expression, and second, prevention of sweat, water, and other debris from falling down into the eye socket. It is common for people to modify their eyebrows by means of hair removal and makeup. A number of theories have been proposed to explain the function of the eyebrow in humans. One approach suggests its main function is to prevent moisture (mostly sweat and rain) from flowing into the eye. Another theory holds that clearly visible eyebrows provided safety from predators when early hominid groups started sleeping on the ground. Recent research, however, suggests eyebrows in humans developed as a means of communication and that this is their primary function. Humans developed a smooth forehead with visible, hairy eyebrows capable of a wide range of movement. Such eyebrows can express a wide range of subtle emotions – including recognition and sympathy. Fashion in eyebrow shape has changed throughout the ages and eyebrows have always featured heavily in female fashion, often as part of cultural demands made on women about body hair. Cosmetic methods over the centuries have been developed to alter the appearance of eyebrows, by adding or removing hair, changing the color, or changing the position to meet the aesthetic ideal of the time, for example, by tinting the eyebrow with permanent dye, similar to hair colour, often in order to darken them. Eyebrow brushes, shaders, and pencils are often used to define the eyebrow or make it appear fuller. These can create an outline for the brows or mimic hairs where there are sparse areas. Brow gels are also used in creating a thicker brow; they allow for the hairs to be more textured, which gives the appearance of thicker, fuller brows. Lastly, brow powders or even eyeshadows are used for those who want a fuller and more natural look, by placing the brow powder or eyeshadow (closest to the natural hair colour) in areas where there is less hair.