Lessons you start as a guest live on this device only. An account keeps them, tracks what you have understood, and opens every model.
Name a topic and it becomes a lesson: an explanation written for where you're starting from, questions that check whether it landed, and a task that makes you apply it. Your answers are marked, and what you've understood is kept.
Why does a recursive function need a base case?
"Because computers are fast."
That is not what a base case is for. It is the condition that stops the recursion — without it each call makes another and the stack runs out.
Same topic, different lesson depending on where you start.
Graded against what the lesson taught, with the gap named.
Every lesson keeps its score, and the days add up to a streak.