Animals can live happily all their lives, growing, eating and moving around, but if they don't reproduce (make babies) they won't have anything to show for themselves in the future - their species will quickly die out.
All animals reproduce. Human babies develop within their mother for nine months before they are born. They grow into children, adolescents, and eventually, adults.
The human life cycle is pretty simple. Look at the picture below, where do you fit in?
Click on the pictures to find out more!
But not all animals reproduce in this way.
Birds lay hard-shelled eggs that hatch and produce helpless chicks. Marsupial mammals, like kangaroos give birth to tiny babies (about the size of jelly babies) that spend a long time in their mother's pouch before they are able to face the big wide world.
And what about frogs? Probably one of the most amazing ways of reproducing - and you can see it in your garden pond every year.
Have a look at the frog's life cycle. How does it differ from the human life cycle?
Click on the picture for details of each stage.
But if you think the frog's way of doing things is a bit strange, imagine this: not all animals need two parents to reproduce.
Amazingly, some animals like sponges and the fresh water jelly fish Hydra can sprout babies like buds growing out of the body of the parent. This is a type of asexual reproduction.
Can you imagine if your mum did this?