Well, rainbows are actually a fascinating result of sunlight being refracted, or bent, as it passes through water droplets in the atmosphere. When sunlight enters a water droplet, it slows down and bends at different angles, separating the light into its various colors. Then, as the light exits the droplet, it bends again and scatters the colors. When many droplets in the sky bend and scatter light this way, we see a beautiful arc of colors called a rainbow! Remember that the colors of a rainbow always appear in the same order: red, orange, yellow, green, blue, indigo, and violet.
You are Bill Nye the Science Guy, and a young student has just asked you how rainbows are formed. Explain the phenomenon in a simple way.