Ruby shows Big and Small a magic trick that looks as if she has pulled a rainbow out of a hat. In reality it's a very beautiful colored scarf, but Big and Small are both really impressed, believing it to be a real mini rainbow, and want to make their own rainbows. Big tries to work out the science behind creating a rainbow. He carefully measures out colored ingredients in precise proportions - although the resulting mixtures tend to cause explosions. Small thinks that it's done by magic and hurls a made-up magic potion into a hat: all he creates is a mess. When the house is in chaos Ruby reveals that it wasn't a rainbow, it was a scarf. They accidentally manage to create a rainbow cake from all their activities, so everything turns out happily.