Scientists know a lot more about cancers now because so much research is going on to find out about it.
But there's still lots to discover.
Because there are so many different types of cancers that can happen anywhere in the body, there are different causes.
What experts do know is cancer is caused by damaged genes in your DNA.
Genes are the set of coded instructions which decide what happens to a cell in its life. The instructions are kept in a cell's nucleus.
But if genes get damaged, this can trigger wrong cell behaviour.
Cells start to behave out of control - like a naughty kid at school - and they ignore the rules on how to behave.
Experts reckon there are three main causes of gene damage:
- Harmful sun rays