In epidemics, the messenger matters: messengers are what make something spread. But the content of the message matters too. And the specific quality that a message needs to be successful is the quality of “stickiness.” Is the message—or the food, or the movie, or the product—memorable? Is it so memorable, in fact, that it can create change, that it can spur someone to action?

— The Tipping Point: How Little Things Can Make a Big Difference by Malcolm Gladwell