As a concept, the Like button ping-ponged around Silicon Valley years before Facebook debuted it in 2009. Notably, designer Bob Goodson sketched “thumbs up” and “thumbs down” buttons for Yelp in 2003 on a hunch that driving engagement on user reviews would encourage more people to post them. He was right: By giving users an easy way to rate a review “useful,” “funny,” or “cool,” Yelp drove an explosion in reviews—and inspired a secret project codenamed “Props” at Facebook.

Today, Like buttons form the backbone of an engagement-driven economy, receiving an estimated 160 billion clicks per day. The simple thumbs-up has evolved into a rich visual lexicon of reactions, from the upvote arrow on Reddit, to the double-tap heart on Instagram, to the sprawling “reactji” palette on Slack. Everywhere we go online, we have access to an instant, universally understood gesture of affirmation or, in some cases, disapproval—and an implicit way to curate the content we consume. The ebb and flow of likes shapes what creators promote, how platforms distribute content, and most importantly, how users gauge relevance.