It involves a man and woman having sex without birth control until the man ejaculates, or releases, semen from his penis into the woman's vagina.
The semen, containing sperm (which is made in his testicles), moves to the Fallopian tubes, and if it finds an ovum on its way, it will try to fertilize it. Certain species of animals also have sex for other purposes than to bear offspring.
Two animals coming together for the purposes of sexual reproduction is called mating.
Most animals only mate when the female is at the point of estrus, which is the most fertile period of time in its reproductive cycle. Forcing someone to have sex who does not want to, called rape, is a serious crime in many countries.
Humans, bonobos and dolphins show cooperative behaviour.
Sex reinforces intimate social bonds between individuals.
Overall, such cooperation also benefits each member of the group in that they are better able to survive.
In humans, sexual intercourse seems to serve three types of purposes,which do not exclude one another: In the late 20th century, very effective forms of contraception (birth control) were developed allowing a man and women to help prevent a baby from being made when they have sex. This is a piece of rubber that covers the penis that a man can wear during intercourse, which stops the man's semen from going into the woman's vagina.
This does not always work though because the condom may rip or tear.