Lessons of a skill.
Lessons in Duolingo are collections of exercises that teach core components of skills.
Description[]
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.
Strength bar lessons[]
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:[1]
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.