Esio Trot by Roald Dahl.
Mr Hoppy is a retired man who is deeply in love with his downstairs neighbour Mrs Silver — but is too shy to tell her. Mrs Silver's great passion is her pet tortoise, Alfie. She longs for Alfie to grow bigger. Mr Hoppy tells her that if she says a magic spell ('Esio trot, esio trot, teet ot teet ot...' — 'tortoise, tortoise, come to me' backwards) then Alfie will grow. Meanwhile, Mr Hoppy secretly buys many tortoises, each slightly larger than the last, and swaps them through Mrs Silver's balcony. Mrs Silver is delighted as 'Alfie' grows bigger week by week. Eventually the truth is revealed — but by then Mrs Silver loves Mr Hoppy so much that she doesn't mind. They marry, and the many tortoises are all given to good homes.