Practise the reasoning behind the code.

In an interview, you need to explain how you reached a solution and defend its trade-offs. When reviewing a change at work, you need to judge whether it addresses the problem and what could still go wrong, whoever wrote it.

These scenarios exercise that reasoning: investigate unfamiliar code, challenge assumptions, verify behavior, and explain your decisions.

Each scenario has two steps. First you fix the bug yourself. Then you review a fix someone else wrote for the same bug and decide whether it really works, before the checks show you.

Work through each exercise without AI assistance so you can see which parts you understand independently and where you need more practice.