Sex is either of two divisions, typically male or female, into which organisms that propagate their species through sexual reproduction are divided. Most plants and almost all animals employ sexual reproduction. Animals are usually mobile and seek out a partner of the opposite sex for mating. Animals which live in water can mate using external fertilization, whereas most land-based animals, such as reptiles, birds, and mammals, including humans, use internal fertilization. Plants are generally immobile, and in seed plants sexual reproduction relies on pollination, either through self-pollination, or via cross-pollination with other plants of the same species.
