This is the twelfth picture book about Frog, and his friends Duck, Hare, Rat and Pig, and certain to charm his legions of admirers with more of his na-ve antics.      Frog is miserable, and he cannot see why. Nobody has upset him, and nothing has gone wrong particularly, but he still feels blue. Then one of his stalwart friends makes him laugh, and suddenly he realises he''s not sad anymore. He still doesn''t know why he was so sad in the first place, but it doesn''t matter. A perfect book to explore emotions and mood swings.