Mungo adores his big sister Clementine and his big sister is very clever. She knows everything and Mungo relies on her for answers to all his questions. How do cats stay cool in the summer? What is in the centre of the earth? How do leaves change their colours? Clementine has some very interesting answers for her little brother - each with a germ of truth. For example, cats take off their coats, and 'pigments' paint the leaves a new colour. But one night it is the big sister that has a question for the little brother, and it is the honesty and sweetness of Mungo's answer that has the real truth.