And between the two is another distinctly human supernormal stimulus: food porn, the carefully arranged, carefully filtered images that show a meal—homecooked or restaurant-served—at its most appealing.

Food porn is defined in part by the senses that it is a visual experience of something that other people can smell and taste. Food porn, as Amanda Simpson, the creator of the site Food Porn Daily, told The Daily Meal in 2010, is “anything that makes me drool”—something that, at its best, should manufacture a desire that it can’t satisfy.

What’s the appeal in ogling what you can’t have? In the case of food porn, at least, researchers still aren’t sure.

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The first documented use of the term “food porn” comes from the feminist writer Rosalind Cowards’s 1984 book Female Desire. It was referenced every so often for the next two decades or so by food writers and chefs, according to the site Know Your Meme, but didn’t take on its current meaning—food photos shared through social media—until the early 2000s. The photo-sharing website Flickr launched a “Food Porn” category in September 2004 (today, it has around three-quarters of a million photos).

And a few months later, in April 2005, it entered the Urban Dictionary lexicon. Definition: “Close-up images of juicy, delicious food in advertisements.” Used in a sentence: Oh, that McDonalds ad was like food porn. I want a Big Mac sooo bad.

Urban Dictionary, though, makes an assumption on something that research hasn’t yet been able to prove. A McDonalds ad for a Big Mac may look delicious, but what’s still murky is whether that necessarily translates into hunger for a Big Mac.

The chef’s maxim that people first eat with their eyes is backed both by common sense—food stylists exist for a reason, and a glistening grill-marked burger that oozes cheese is an easier sell than a limp, gray one—and by science. A 2012 study published in the journal Physiology and Behavior, for example, found that seemingly minute details about a dish’s appearance, like “gloss, evenness, and shape,” can alter how diners perceive its taste and smell.

But what happens when eating with the eyes is the only step, rather than just the first—when the image isn’t a bridge to smelling and tasting a dish, but the entire experience?

Some scientists believe—like Simpson—that images of food only trigger the desire for the real thing. A 2012 study, for example, found that just looking at pictures of food may be enough to cause an uptick in ghrelin, a hormone that causes hunger.

One reason may be that looking primes the brain for eating. “If you think about throwing a baseball, your brain reacts like you’re really throwing a baseball,” explained Gabriella Petrick, a professor of nutrition and food studies at George Mason University. “When we eat things, different parts of our brain light up in different ways. It’s not just taste—we evoke sight, we evoke hearing, we evoke lots of different [things] as our brain tries to construct what our food is.”