Humpty Dumpty was Lewis Carroll’s first annotator, but he wouldn’t be the last. When Alice meets Humpty, about halfway into Through the Looking-Glass and What Alice Found There, he’s fairly obnoxious, rudely telling her she has “no more sense than a baby.” When she admits to not knowing what he means by the phrase “There’s glory for you!,” he replies, “Of course you don’t—till I tell you.” Nevertheless, Alice courteously asks him for help in deciphering a strange poem she has encountered earlier in her travels:

“You seem very clever at explaining words, sir,” said Alice. “Would you kindly tell me the meaning of the poem called ‘Jabberwocky?’ ” “Let’s hear it,” said Humpty Dumpty. “I can explain all the poems that were ever invented—and a good many that haven’t been invented yet.”

In the ensuing pages, he offers glosses on puzzling words like slithy (“‘lithe and slimy’…You see it’s like a portmanteau—there are two meanings packed up into one word”) and outgrabe (“something between bellowing and whistling, with a kind of sneeze in the middle”).

Some might describe Humpty’s patronizing, pompous tone as mansplaining (or maybe, given the source, “eggsplaining”). But plenty of readers will understand the urge to provide definitions and interpretations as well as Alice’s desire to hear them. In recent years, more and more of us are reading annotated editions of our favorite books—The Annotated Wuthering Heights, The Annotated Lolita, The Annotated Anne of Green Gables—as well as posting on sites like Genius.com, which claims to host more than a million annotated texts, and sharing notes and highlights on Kindle. Never before has there been so much activity in the margins of culture.

What makes a good annotator? It’s some combination, apparently, of excess and restraint: an instinct for when to tell us more than we need to know (or more than we knew there was to know) balanced with a refusal to bore us. The obvious is, obviously, out: Most readers of Carroll probably won’t need to be told what croquet is, for instance. More difficult is distinguishing between an alluring obscurity and a total waste of time. Not all rabbit holes are worth going down.

This is a question that Genius, whose slightly megalomaniacal slogan is “Annotate the world,” is still grappling with. The site launched in 2009 as Rap Genius, devoted to the explication of rap lyrics. These origins have been the occasion for a lot of dismissive jokes, but it makes perfect sense that an annotation web site would orient itself toward such a dense, allusive art form. Rap, like Carroll’s writing, is full of mysteries that cry out for explanation: Think of the Wu-Tang Clan’s mix of Five Percenter lingo, references to kung fu films, and drug trade slang, for instance, or Kendrick Lamar’s brash collages of personal confession and black history. It’s also, as with children’s literature, an often disrespected genre that inspires a passionate devotion in highly intelligent people who are ready to sound the depths of their pleasures in order to prove the skeptics wrong.