I was doing housework - spraying vinegar mixed with water on everything - and when I sprayed this chair with a beam of sun shining strongly on it, the water appeared to reveal a rainbow every time I sprayed. It was so beautiful! Then I remembered the activity from a meteorology (weather) book, which was exactly what I had just done - but I hadn't tried it. Try it with the kids!
Where do rainbows come from? "Each raindrop splits the sunlight into the colours of the spectrum." (Meteorology, by Graham Peacock.)