In the seventeenth century, both pots and kettles turned black because they were used over open fires. Today, this idiom means criticizing someone else for a fault of one's own. Raining cats and dogs. In Norse mythology, the dog is associated with wind and the cat with storms. This expression means it's raining very heavily.
Crocodiles have a reflex that causes their eyes to tear when they open their mouths.
This makes it look as though they are crying while devouring their prey.