Sexual intercourse (or simply called sex) is the insertion and thrusting of a male's penis into a female's vagina. People and animals that sexually reproduce use sexual intercourse to have an offspring. Sometimes sexual intercourse is called coitus or copulation and is more casually known as having sex or sleeping together. The two animals may be of opposite sexes or they may be hermaphroditic, as is the case with snails. Sexual intercourse may also be between individuals of the same sex.