`Mary, what are you doing? We can''t go in here. This is someone''s house.'' Sometimes he couldn''t believe the things his sister did. She was always getting them into trouble-and he usually had to get them out.Twins, Andy and Mary, are in Amsterdam with their mother. But they see something very strange when they decide to explore an empty house. Will anyone believe them before it''s too late?Cambridge Discovery Readers is an exciting new series of original fiction, adapted fiction and factbooks, specifically written for pre-teen and teenage learners of English. Graded into seven levels-from Starter to Advanced-the readers provide easy and enjoyable reading with skills and vocabulary practice on a wide range of contemporary topics and themes.