There is a specific bug that teaches everyone about keys. You have a list of stateful rows — each with a checkbox, or an expansion tile, or a text field. You delete the second row. The row disappears correctly, but the checkmark that belonged to it is now sitting on a different row. Nothing about your data is wrong. Reload the page and everything is fine. That bug is not a Flutter defect. It is the framework doing exactly what it was told, and the fix is one word long. But the fix only sticks if you understand the rule underneath it, because the same rule explains why keys sometimes do nothing at all, why GlobalKey is expensive, and why a PageStorageKey is not really a key in the same sense. Three trees, and the one that holds your state Flutter maintains three parallel structures. The widget tree is your build output: immutable configuration objects, thrown away and recreated constantly. The render tree does layout and painting. Between them sits the element tree , and that i...