Duolingo is a popular language learning app built around short daily lessons, and finishing one triggers a short celebratory sound in the app itself. The green owl mascot, Duo, became a widespread internet joke about reminding users to keep their streaks, with memes warning that the owl will come for anyone who skips a lesson. That joke helped the completion sound travel beyond the app, as creators began using it in study videos, reaction clips, and memes to signal a job done.