The Test Drawer
This video presents the same text shown beside it, spoken and on screen. It adds nothing the text does not say.
State
Before any learning begins, part of the data is locked away as a test set — untouched until grading day, because a test row that influenced learning is training data under another name.
Show
The stall log: rows one through six teach; rows seven and eight wait in the drawer.
Watch for
Peeking converts test into training silently — the drawer's value is exactly its untouchedness.