Learn a software feature with a tiny project

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I want to learn a software feature with a tiny project. Create a small exercise that uses the feature and makes errors visible.

Use these inputs:
Feature: [Add feature]
Current knowledge: [Add current knowledge]

Create: Guided exercise.
Separate supplied facts from assumptions. Ask only for missing information that changes the result.
Make the result specific enough to use and include a short way to check or improve it.