Identify the grammatical error in the phrase.
It's raining cats and dog.
The grammatical error in the phrase is that the idiom "It's raining cats and dogs" has been altered. Specifically, the word "dogs" is wrongly in singular form as "dog". The correct sentence should be "It's raining cats and dogs."
Complete the following task based on the instruction and optional input provided.### Instruction:
Identify the grammatical error in the phrase.

### Input:
It's raining cats and dog.

### Output:
The grammatical error in the phrase is that the idiom "It's raining cats and dogs" has been altered. Specifically, the word "dogs" is wrongly in singular form as "dog". The correct sentence should be "It's raining cats and dogs."