I have this sentence.
It rains dogs and cats
I didn't know the meaning of it, until someone told me that it means "it rains a lot."
Now I have this sentence.
If we examine porpoises and dolphins at this level, they are as physically different as dogs and cats.
What does dogs and cats mean in this other sentence?
Is there any other usage of "dogs and cats"?
The expression is '... raining cats and dogs'. I have never heard of it raining dogs and cats.
But this is an expression, which a) is not meant to be taken literally and b) only holds when the 'cats and dogs' are taken as one (grouped) item
'... are as physically different as dogs and cats'.
In this sentence, the dogs and cats are not grouped, but are being referred to individually and separately. In addition, they are being referred to literally. In this context the phrase 'dogs and cats' is referring to two different four-legged, hairy mammals that people keep as pets.
The point the author is trying to make is that dolphins and porpoises (two very similar appearing animals) are actually quite different and that you should consider them as being as different to each other as much as you would consider dogs and cats as being different to each other. As @kojiro points out, in this context, the order of dogs and cats does not matter, yet as has been mentioned raining cats and dogs is always ordered cats, then dogs.
As an interesting aside, it seems other versions of it's raining ... for instance 'it's raining money', 'it's raining men', 'it's raining lawsuits', do refer to an abundance of the object - unlike raining cats and dogs - but the rain may be figurative. I suppose in that context - perhaps on entering an animal rescue shelter - you could say 'it's raining dogs and cats'.
First, the idiom is "It's raining cats and dogs!"
The answer to your first question is that cats and dogs are much bigger and heavier than normal raindrops, so the rain being described is much heavier than normal. It's not just "raining" but it's "super-raining" or "hyper-raining" or "ultra-raining".
Imagine a hailstorm with hailstones as large as baseballs: get hit by one and you'll probably die.
For the second question, dogs and cats are extremely different despite their being four-legged mammals that people keep as house pets. Porpoises and dolphins look like fish but are mammals, and, although to some people they might seem quite similar, they are actually as different as dogs and cats, the sentence says. I suppose the author might as well have said "as different as chickens and penguins" (both medium-sized essentially flightless birds) or "as sparrows and hummingbirds" (both very small birds), but most people are more familiar with cats and dogs than the other examples I gave because cats and dogs are ubiquitous (so are chickens) but sparrows, hummingbirds, and penguins aren't.
Porpoises and dolphins are animals, dogs and cats are as well. The actual animals are meant in the context provided, because the text compares these animals.
A big clue whether or not real dogs and cats are meant is the part "it rains" in your first example. Dogs and cats don't suddenly rain from the sky, so it is used in the figurative sense in your first example
My two cents:
While other answers talk about what is that idiom, I had read somewhere (not sure where) that it came from the practical observation many years back.
When it rained heavily, the cats and dogs walking on the roof then started falling down. These animals generally don't fall (as they have a perfect grip while walking), but then the rains were so heavy, they would lose their balance and start falling from the roofs. That's why it came from as a metaphor that it rains so heavily that cats and dogs will fall if they are on the roofs.
I. It's raining cats and dogs - Origin
Ref. The Guardian
It is thought that the phrase originated in England during the seventeenth century. During heavy rains, floods washed dead stray cats and dogs across the streets. https://www.theguardian.com/notesandqueries/query/0,5753,-22408,00.html#:~:text=The%20phrase%20is%20supposed%20to,ancient%20associations%20with%20bad%20weather.
II. Dolphins and Porpoises = Cats and Dogs
If we examine porpoises and dolphins at this level, they are as physically different as dogs and cats. What does dogs and cats mean in this other sentence?
The world is divided into two groups: cat lovers and dog lovers. Both creatures have completely distinct traits, thus the author of the above phrase utilised this to demonstrate the dissimilarity between a dolphin and a porpoise. You'll be able to grasp this after reading the sayings in the next topic.
III. Other proverbs on Cats and Dogs.
Is there any other usage of "dogs and cats"?
I would be surprised if there weren't any.
Ref. pethelpful.com.
1.Dogs remember faces, cats' places..
An English proverb. It perfectly describes the difference between the canine and the feline mentality.
2.The dog for the man, the cat for the woman.
This English proverb that says more about people than it does about the dog and the cat.
4.The dog may be wonderful prose, but only the cat is poetry..
This French proverb is very elegant in its description.
5. In the long run, even a dog will compromise with the cat..
This Hungarian proverb recognizes the superior creature.
6. Dogs believe they are human. Cats believe they are God..
People like to humanize their dogs. It seems like the cat has higher aspirations.
7.Dogs have owners; cats have staff.
No point in arguing. We all know it. We own our dogs while our cats own us.
8. Having done wrong, the cat will always try to make it look like the dog did it..
Sneaky cats know how to do bad things and make you think someone else did it.
9.Buy a dog a toy, and it will play with it forever. Buy a cat a present, and it will play with the wrapper for 10 minutes..
A true statement. Try it for yourself. A cat will ignore the toy but will play with the box it came in.
10.Every dog has its day—but the nights are reserved for the cats..
The night and darkness are synonymous with cats.
https://pethelpful.com/cats/77-Cat-Proverbs-And-Sayings#:~:text=44.,the%20dog%20and%20the%20cat.
My copy of Brewer's Dictionary of Phrase and Fable has quite a lengthy explanation for "raining cats and dogs", although none of it is definitive:
'In northern mythology the cat is supposed to have great influence on the weather, and "The cat has a gale of wind in her tail" is a seafarers' expression for when a cat is unusually frisky. Witches that rode on storms were said to assume the forms of cats. The dog is a signal of wind, like the wolf, both of which were attendants of Odin, the storm god. From these facts it has been speculated that the cat may be taken as a symbol of the pouring rain, and the dog of the strong gusts of wind accompany a rainstorm - whence "rain cats and dogs". However, it is also the case that the descent of various bizarre objects (e.g. pitchforks and chicken coops) from the sky has been cited in English and other languages as evidence of abnormally heavy rainfall, and it may well be that cars and dogs are simply two further examples of this.'
The verb "to rain" can be transitive. "To rain X" basically means that "X is falling out of the sky", whatever X is.
For instance, "On Saturn's moon Titan, it rains methane."
"Raining cats and dogs" literally means that small animals are falling out of the sky. But, of course, this image of animals falling from the sky is a metaphor for very large, heavy drops of water (and possibly dark skies, since animals are opaque).
The phrase is not an idiom, as the other answers misinform you. An idiom occurs when some piece of bad grammar is given an accepted meaning ("catch as catch can"), or when some sentence or phrase is given a completely different meaning from what it appears to say at face value ("kick the bucket").
"A and B are as different as X and Y" means that A is different from B, and this situation between A and B is similar to the situation between X and Y, which are also different from each other. The differences between A and B can be similar or parallel to those between X and Y, or they might not be. If A and B are very similar, then we actually expect X and Y to be similar.
Examples:
"Classical music and grunge rock are as different as day and night". In this case, we learn nothing from the analogy, except that the speaker feels very strongly about how different those two kinds of music are. Two kinds of music are different in ways that do not really correspond to the differences between day and night.
"Porpoises and dolphins are as different as cats and dogs". Here, porpoises and dolphins are actually similar, but they have differences. Cats and dogs are also similar, and have differences. Those differences are somewhat parallel to those between porpoises and dolphins.
