The animated GIF above—along with the caption "Look at all those ducks there are at least ten"—is currently floating around the Internet. The thousands of people who have liked and shared it on various sites, including Tumblr, Reddit, and Imgur, presumably think it's entertaining, but why?

At the risk of spoiling the joke by overanalyzing it, a linguist named Paul Grice has an answer.

What's curious, at first glance, is the contrast between "at least ten ducks" and the vast swarm of ducks in the actual picture. The incongruity of the caption leads to humor, right? But this too is strange. After all, the statement is technically true, as pointed out by the almost-as-popular follow-up comment: "Well, you're not wrong." And sure enough, if we were sorting a series of images into piles of those containing at least ten ducks and those containing nine ducks or fewer, this throng of ducks would no doubt fall into the first group. Why, then, does it sound funny to make—albeit out of the blue—what is undeniably a true assertion? Is it simple understatement, or is there something more at work?

Here's where Paul Grice comes in. Grice was the first to describe some of the implicit assumptions that everyone makes in conversation, known as Gricean Maxims in his honor. In particular, he noted that everything that someone says is interpreted as if it were relevant to the circumstances and what had been said before. It sounds simple, but it's something that computers, compared to humans at least, are surprisingly bad at.

Take the following exchange, for example:

Speaker 1: "Would you like some coffee?"
Speaker 2: "Coffee would keep me awake."

Speaker 1 could reasonably interpret that answer as a yes or a no depending on the context. If it's morning and there's work to be done, being kept awake is likely desirable, but if it's late evening, well, probably not. It all hinges on what is the most relevant interpretation: The yes or no isn't inherent to the answer itself, but you figure it out, without much thought, based on your common-sense knowledge of the world, referred to in linguistics as your pragmatic competence.

Grice and his successors break this idea down further. In general, we assume that people are truthful, that they are giving as much detail as is pertinent and no more, and that they're being as clear as possible. When someone seems to be violating one of these assumptions, it's logical to deduce that they're doing so for a particular reason, such as biting sarcasm or, in the case of our ducks, for laughs.

Which of the following statements conveys more complete information?

1. "I ate some of the cookies"
2. "I ate all of the cookies"

Obviously, the latter provides you with a more definitive picture of the current state of the cookie jar. You would expect me to have pretty reliable knowledge of my own eating behavior, so if I said that I ate some of the cookies, I must be implying that I did not, in fact, eat them all. Otherwise I would have said so.

The technical name for this type of reasoning is scalar implicature, and it's precisely what's going on with the ducks. Since we expect that the caption will give as much information as is relevant, we might assume that ten is a reasonably close estimate of the actual number of ducks in the picture. But it's abundantly evident that there are more than ten. So a statement that there are at least ten ducks in the picture is out of whack with what we see: On the one hand, the writer of the caption couldn't possibly believe that there are maybe ten, maybe a few more, ducks in this picture, but on the other, the Gricean Maxim related to quantity that normally works so well suggests that this is in fact precisely what the writer believes. And yet the caption remains incontrovertibly true in the most literal sense. Just as with a classical paradox or an optical illusion, puzzling over why we can get multiple interpretations from this caption in this context can cause surprise and entertainment. Similarly, I could play with your expectations by saying:

"I ate some of the cookies … [pause] … actually, I ate all of them."

Providing incomplete information up front—being temporarily misleading, in effect—turns a matter-of-fact statement about eating all of the cookies or seeing some number of ducks into a punchline.

Of course, as with any type of humor, implicatures can be overplayed, so one of the fun things about Gricean Maxims is that they explain why certain people are really annoying. We've all encountered that guy who, when asked "Do you know what time it is?" will reply "Yes." He's responding to the literal meaning of the question and not to its larger, pragmatic implications.