Duolingo turned language learning into a game — and turned a green cartoon owl into one of the most recognized mascots in consumer software. The visual system is built on a single saturated green, chunky rounded geometry, and Duo himself in dozens of expressions from cheerful encouragement to passive-aggressive guilt. Everything is round, everything is loud, and the aesthetic reads like a Saturday-morning cartoon UI rebuilt for adults with streaks to protect.

The design system codifies that energy: solid-bottom-edge press-shadow buttons, circular avatars with thick borders, speech bubbles with rounded tails, and a children's-book color palette cranked past comfortable saturation. No minimalism, no muted tones, no sharp corners. The owl is watching.