If you want to know the origin of this idiom, read it on this website: http://www.phrases.org.uk/meanings/raining%20cats%20and%20dogs.html
Better an Oops than a What if...
ACE THE IELTS WITH EASE
Activities, games and resources for teachers of young learners
L'osservatorio di Fractals su Fashion & Lifestyle
Instructional Design works of Ridvan Saglam