The difference between the almost right word and the right word is really a large matter—'tis the difference between the lightning-bug and the lightning.

Twain repeated the lightning bug/lightning comparison in several contexts, and credited Josh Billings for the idea | Josh Billings defined the difference between humor and wit as that between the lightning bug and the lightning. | Speech at the 145th annual dinner of St. Andrew's Society, New York, 30 November 1901, Mark Twain Speaking (1976), ed. Paul Fatout, p. 424 | Billings' original wording was characteristically affected | Don't mistake vivacity for wit, thare iz about az mutch difference az thare iz between lightning and a lightning bug. | Josh Billings' Old Farmer's Allminax, "January 1871". Also in Everybody's Friend, or; Josh Billing's Encyclopedia and Proverbial Philosophy of Wit and Humor (1874), p. 304