Our planet is the only planet in the Solar System where rainbows are possible. They are not visible in big cities, due to pollution and smog. But if you are travelling after rain has fallen in the countryside or walking on the beach, you have a good chance to see a rainbow. The rainbow is a multi-coloured arc which appears in the sky. When you see a rainbow, the sun is always behind you and it is raining in front of you. It is impossible to get to the end of the rainbow and to touch it. When you move, the rainbow moves too. We have always learned that rainbows have 7 colours. However, until the 17th century, people used to think there were only 5 colours in the rainbow.