Sex is not just a part of life it is the process by which it is created. Sex, obviously, involves both a male and a female. The female provides the egg[i] and the male provides the sperm. The fusion of the two nuclei (the plural of nucleus) is called fertilisation. After fertilisation, the fertilised egg becomes what is known as a zygote[ii].        

In the tables below, we have summarised all of the main parts involved in sexual reproduction:

There are two different types of fertilisation: internal and external. Internal fertilisation is where the penis is instead into the female’s vagina and the eggs are fertilised within the female’s body. External fertilisation on the other hand is where the egg is fertilised outside of the body. For something to be fertilised out of the body, it needs something to swim in. This is the reason why organisms that use external fertilisation live in water.                   

Now we’re going to explain what happens step-by-step in sexual reproduction in humans (and other mammals[iii]). These are the stages of sexual reproduction:

1.      The penis is inserted into the vagina.

2.      Intercourse.

3.      Sperm is released form the testis, goes through the urethra and then up the sperm duct.

4.      Ejaculation.

5.      The sperm swims up the vagina to the oviduct where an egg has been released from the ovary.

6.      The nucleus of the sperm and egg fuse forming a zygote (fertilised egg).

7.      The egg moves to the uterus and implants itself.

The sperm is well suited to its role it fertilisation. Human males typically have millions and million of sperm cells at any one time. These sperm cells are absolutely tiny at around 40 microns (forty millionths of a metre) in length. Below is a basic diagram showing a sperm cell:

[ii] Sorry to spoil the ending of the dictionary for you. 

[iii] A mammal is an organism that gives birth to live young. PS. Stuff the platypus, it’s just weird.