Trains are great dirty smoky things," said Will. "You won't like it." Tessa was unmoved. "I won't know if I like it until I try it, will I?" "I've never swum naked in the Thames before, but I know I wouldn't like it." "But think how entertaining for sightseers," said Tessa, and she saw Jem duck his head to hide the quick flash of his grin.