Cancer is a nasty disease which kills millions of people every year. It starts when something goes wrong with cells in an animal's body (other animals get cancer too) which makes them start dividing uncontrollably. This makes a mass of new cells called a tumour which just grows and grows. Normal cells divide a few times and then stop. Tumour cells just keep on dividing and growing. They can block or crush other vital organs inside your body causing pain and, eventually, death. Cancerous tumours (also called 'neoplasms') can also shed cells into the bloodstream. Doctors call this 'metastasis'. These cells end up making more tumours in other parts of the body making death virtually certain.

Humans spend billions on health services to try and stop cancers. Many people feel that it would be better if everyone did their best to try and prevent cancers growing in the first place rather than attempting to treat them after they've started. If kids never started smoking, for example, most of them would live much longer and healthier lives. And when you think about it, there is nothing more important to you than good health. Eating good food is a great way to make sure that your body grows and stays strong and healthy — and so is taking plenty of exercise.