Ready for some pig heart and stomach on a bun? Not salivating yet? I’ll add BBQ sauce and pickles.

Still not hungry? You might be surprised that it’s been a success for McDonald’s. Of course, they don’t call it barbecued heart, stomach, and tripe. They call it McRib. And it’s a perennial top-seller for the fast-food chain. But when McRib appeared on the scene, the sandwich formed from bits of pork pressed into a rib-like configuration wasn’t exactly a hit.

In Jonah Berger’s great book on the psychology behind what makes things go viral, Contagious, he shares the secret to McRib’s eventual success. It’s a concept you may remember from Economics 101: scarcity. Most of the time, McD’s doesn’t offer McRib to its customers. When it does, it’s for a limited time, usually in a defined area. Those who crave the porcine concoction eagerly await its arrival and rush to the restaurants to buy it.

Fan have embraced a whole-hog adoption of social media to track the latest appearance of the faux ribs. Facebook and Twitter feeds dedicated to McRib have given birth to an online tracker. “Making people feel like insiders can benefit all types of products and ideas,” Berger concludes, “Regardless of whether the product is hip and cool, or a mix of leftover pig parts.”