Daughter's Questions: Rainbows

This is the first post in a series.

Our young daughter asks lots of difficult questions, and while I think we do quite well answering them, I'd be interested to know how others would have answered the same questions. To stop answers getting muddled up together, I'll just put one question in each post.

Our daughter is currently six years old, and recently asked something along the following lines:

If a raindrop is like a prism and can make a mini-rainbow all on its own, how come we see one big rainbow in the sky and not lots of little ones?