Cotton is, of course, one of those oppositely charged molecules. Cotton's lengthy cellulose molecules include oxygen-hydrogen groups that attract water, pulling it straight up into its fibers. Cotton is thus classified as a hydrophilic substance. This means it attracts water, which makes it useful for clothing and other products where moisture absorption is desired. The term "hydrophilic" is used in science to describe any molecule or material that attracts water; the opposite is "hydrophobic" (or water-repelling). So, cotton is hydrophilic because of the presence of hydroxyl groups attached to its cellulose molecules. These hydroxyl groups attract water, causing it to come together in small droplets on the surface of the cotton fiber. When cotton is woven into fabrics, the hydrophilic nature of the fiber causes water to be attracted to the surface of the fabric, which can lead to water damage. This is why water-resistant fabrics such as polyester have become popular alternatives for clothes that are exposed to rain or other forms of moisture. Cotton is also hydrophilic when it comes in contact with other substances that contain hydroxyl groups, such as glue, nail polish, and toothpaste.