A Girl Called Harry

Harriet Jasmine Emerald Florence Mabey McDonald (Harry for short) wonders if an imagination is something fast, like a cheetah...in which case it is not your fault if it runs away with you, because you couldn't stop it. Or is it something little, like a celebrity's handbag dog, which can only run away with you if you let it...
When the new girl starts at school, and takes over Harry's best friend, her imagination runs riot.
Perhaps it is more like a kite, which can carry her away...