Raining cats and dogs (idiom)
The origin of this idiom was actually from the dead cats and dogs that washed away by the flood caused by the heavy rain; it looked like like it had just rained cats and dogs.
This idiom is a bit outdated actually; native speakers rarely use it anymore. Instead, you can say “It’s bucketing down outside” or “It’s chucking it down”. However, the idiom raining cats and dogs is still used and taught in non-English-speaking countries.