The female genitalia are mostly internal. The external labium (lips) of the vulva occupies approximately the same physical location on the body as the scrotum would on a male. In the wintertime, when the female enters estrus, the vulva becomes hot and puffy (hence the term "in heat"). This puffy/swelling makes the female visually interesting to males, and along with the pheromones produced in the vagina, indicates when the bitch is physically ready to mate. The puffy vulva is also ideally shaped so that the tip of the male's penis can easily hook into its fold to be directed into the vagina during copulation.