Start with a single fact.

In the margins of some medieval manuscripts — beautiful, expensive books, copied by hand, often full of prayers and psalms — there are drawings of knights fighting snails.

Not dragons. Not lions. Snails.

The knights are in full armour, with lances, swords and shields. Sometimes they charge bravely. Sometimes they kneel and beg for mercy. Sometimes the snail appears to be winning.

That's the fact. Now let's go down the hole.

Level 1: How common is this?

More common than you'd think.

In 1962, the art historian Lilian Randall published a study called The Snail in Gothic Marginal Warfare. She had found around 70 examples of knight-versus-snail combat in some 29 manuscripts, most of them made within a few decades around the year 1300, largely in northern France and Flanders.

So this wasn't one bored scribe's private joke. It was a trend. A meme, seven centuries before the word existed.

Level 2: But why snails?

Nobody left a note explaining it. So scholars have argued about it ever since.

The Lombard theory. Randall's own suggestion was that the snail represented the Lombards — northern Italians who were prominent as bankers and moneylenders in France at the time, and who were stereotyped (unfairly) as cowardly. In some texts the snail sits right next to passages mentioning Lombards. A snail retreats into its shell; a "cowardly" Lombard might be mocked the same way.

The resurrection theory. Others point out that snails disappear into their shells in winter and emerge again in spring, and that the snail could be a symbol of the Resurrection — though that's harder to square with being attacked by knights.

The cowardly knight theory. Perhaps the joke isn't on the snail at all. A knight terrified of a snail is a knight who isn't very brave. In an age that idealised chivalry, a picture of a knight kneeling before a garden pest is pure satire.

The social-climbing theory. Some have read the snail as a symbol of the lower classes, rising up — slowly — against the nobility.

Probably the honest answer is: different artists meant different things, and some of them simply copied a joke that was going around because it was funny.

A knight terrified of a snail is a knight who isn't very brave — and that may be the whole joke.

Level 3: The snails have company

Once you start looking at medieval margins, snails turn out to be the least strange thing there.

These playful margin pictures are often called drolleries, and they include:

• Rabbits hunting humans, sometimes armed with bows and axes.
• Monkeys dressed as bishops, doctors and knights.
• Hybrid creatures — half human, half bird, half fish, often with faces on their bottoms.
• People playing musical instruments that are, on closer inspection, made of other people.
• Behaviour so rude that some 19th-century collectors scraped or painted over it.

This was the "world turned upside down" — a medieval comic tradition in which the weak defeat the strong, animals behave like people, and the normal order of things is turned on its head.

Level 4: Why put jokes in a holy book?

This is the real puzzle. Many of these drawings appear in psalters — books of psalms used for prayer — and in religious and legal texts. They were expensive, carefully commissioned objects.

So why fill them with farting monsters and murderous rabbits?

Level 5: Then the snails disappear

Here's the strangest part of the rabbit hole.

Around the middle of the 14th century, the knight-and-snail motif fades away almost as suddenly as it appeared. It had a lifespan of a few decades — a fashion, like any other.

The bottom of the hole

Here's where we land.

Medieval people are often imagined as solemn and humourless, kneeling in cold churches, fearing God and plague. Their books tell a different story. In the margins of their holiest texts, they drew rabbits jousting, monkeys playing bagpipes and knights losing fights to snails.

The snails were never really about snails. They were about the most human thing of all: the urge to doodle something ridiculous next to something serious, and hope someone else gets the joke.