Maybe it can’t really rain cats and dogs, but it has rained fish and frogs.

While raining cats and dogs isn't literally possible, it's not for the reason you might think. It actually can — and does — rain animals, but only aquatic creatures, which eliminates cats and dogs from the realm of possibility.

Animals literally raining down from the sky is certainly not a common occurrence, but it does happen on rare occasions. And when it does, it’s a real headlining event. Like in December 2021, when fish rained down from the sky in Texarkana, Texas, Newsweek — and many, many other news outlets — reported. 

The phenomenon of animal rain isn’t well understood, but it’s been reported for centuries, albeit rarely, according to National Geographic. When it does occur, it’s typically associated with waterspouts or updrafts. These develop over water, which is why animal rain mainly involves aquatic animals and not the idiomatic cats and dogs.

Waterspouts develop when violent storms churn over large bodies of water, National Geographic reports. These whirlwind clouds then dip into the water, pulling up water and small objects like pebbles and sometimes small animals like fish. Similarly, strong updrafts can sweep small animals up into their vortices. Because of the strength of updrafts, they can sweep up larger animals, such as frogs, snakes, birds and bats, than waterspouts typically do.

When these waterspouts and updrafts move over land, they lose some of their energy, and that’s when animal rain can occur, National Geographic reports. Heavier objects will fall first, and the lightest objects — normally raindrops — will fall last. This explains why when it rains animals it’s typically only one kind of animal. The waterspout or updraft will first drop its heaviest objects, typically frogs or fish in the case of animal rain, then go on to drop anything lighter before finally letting water — in the form of raindrops — fall to the surface.

So if it’s never really rained cats and dogs, why do we say “it’s raining cats and dogs” in reference to a heavy downpours? No one really knows for sure, but there are a few theories, Grammarist reports. The oldest documented use of the phrase is a 1651 collection of poems by Henry Vaughan that includes the phrase “dogs and cats rained in shower.” In 1738, author Jonathon Swift wrote, “I know Sir John will go, though he was sure it would rain cats and dogs.”

One theory about the origin of the phrase links it to 17th-century London. At that time, the city was filthy, and the streets were filled with stray animals, according to Grammarist. When the streets flooded after heavy rains, it’s likely dead animals, including cats and dogs, would be seen floating down the streets, giving the impression that these animals had rained down from the sky.

Another theory about the phrase’s origins dates it much further back in history, to English and Norse mythology. Witches feature prominently in English mythology, with cats heavily associated with them, Grammarist reports. And in Norse mythology, Odin travels in storms with packs of wolves and dogs. These depictions are thought to have possibly given rise to the now common idiom.