Animals of different species rarely get it on, and when they do, they rarely produce offspring.

But even rarer than the ligers, zonkeys, camas, and beefalo of the world are hybrid animals capable of making babies of their own.

In fact, the idea of a mule, the hybrid progeny of a female horse and a male donkey, having its own baby is so implausible that the ancient Romans used to employ the saying, when a mule falls, as a metaphor for the impossible.

The same way that Americans say, when pigs fly, Turkish people say, when fish climb trees, and Nigerians say, when the chicken has teeth.

Here's why.

In order to successfully reproduce, animals need to create viable sex cells.

Normal body cells have two copies of each chromosome, one from the animal's mom and one from dad, sort of like two full decks of cards.

And both decks can simply be duplicated and then split off to form a new cell.

But sex cells are different.

They're produced by taking each set of chromosomes, duplicating them, and then swapping a bunch of cards between decks before breaking them up to form four cells, each with just one complete deck of cards.

This card swap works when the decks in each set come from a mom and a dad of the same species, because they have all the same cards in the same order.

So genes for eye color get swapped with other genes for eye color, and stripe pattern genes get swapped with stripe pattern genes, giving rise to perfectly healthy sex cells.

But in the cells of hybrid animals, the decks aren't identical because they came from parents of different species.

So an eye color card might get swapped for a paw size card, or a bone-making gene for a kidney-making gene, producing two really weird decks that give rise to totally non-functional sex cells.

However, for reasons we really don't understand, it is sometimes possible for a hybrid animal to skip over the card swapping step and sort of just get rid of the dad's chromosomes to create new sex cells that contain only the mom's DNA.

We've actually only seen this happen in female mules.

There are a handful of documented cases in which mules have made babies from eggs containing only their mother's DNA.

Bizarrely, this means that each female mule's offspring was genetically also her half-sibling.

So while, as far as we know, pigs never fly, every once in a great while, a mule foals.