Making colours
A beam of sunlight contains all the colours of the rainbow. When it shines onto something with lots of narrow lines, like a CD, the different colours are separated, so that they can be seen. This is called diffraction. Some birds and butterflies get their colour in this way.

What makes a rainbow?
Rainbows appear when the Sun is shinning and there is rain in the air. The falling drops split the Sun's light into seven different colours, and reflect it towards your eyes. The rainbow you see is always a 'private' one, even if someone is standing close to you, they see a rainbow made by a different set of raindrops. No matter how hard you try, you can never see a rainbow from the side or from behind, and you can never get to the rainbow's end. If you move, the rainbow moves too.