One must always be careful of books,' said Tessa, 'and what is inside them, for words have the power to change us.' 'I'm not sure a book has ever changed me,' said Will. 'Well there is one volume that promises to teach one how to turn oneself into an entire flock of sheep-' 'Only the very weak minded refuse to be influenced by literature and poetry,' said Tessa, determined not to let him run wildly off with the conversation.