A lesson consists of a series of exercises that, when completed, yields XP. The number of exercises in each lesson varies according to the number of questions answered correctly: for each incorrect answer, additional exercises must be completed.
Skills can contain as few as one lesson or as many as ten.
Strength bar lessonsEdit
In the past, lessons were based on a heart system, where a heart was lost for each incorrect answer. This system was progressively replaced on Duolingo's various platforms in 2015 with the new strength bar mechanism, as described by Luis von Ahn:
The goal of a lesson is to fill up a strength bar. When you answer exercises correctly, the strength goes up, and when you get them wrong, the strength goes down.