Challenge 2: Reshaping Wide Monthly Columns into Tall Format — Solution Walkthrough

The correct transformation: Unpivot Columns.

Steps:
  1. In the Power Query Editor, select the Product column (the one
     column that should stay as-is).
  2. Right-click one of the month columns (Jan, Feb, Mar, ... Dec),
     or select all of them, and choose "Unpivot Columns" (or, more
     robust for future new columns: "Unpivot Other Columns" after
     selecting just Product, which unpivots everything else
     automatically).
  3. Power Query replaces all 12 month columns with two new columns.

Reshaped output's columns:
  Product | Attribute | Value

  (Power Query's own default names — typically renamed afterward,
  e.g. "Attribute" to "Month" and "Value" to "Sales", both simple
  rename steps that get added to the Applied Steps list.)

Resulting shape: instead of one row per Product with 12 separate
month columns, there is now one row per Product-Month combination —
a Product that used to occupy a single row with 12 sales figures
across it now occupies 12 separate rows, each with just that one
month's figure in the Sales column.

Explanation:

This is exactly this chapter's own description of Unpivot Columns as
"often the single most useful reshape for badly-formatted source
exports" — wide, one-column-per-period layouts are extremely common
in real-world exports (especially anything originally designed to be
read by a human in a spreadsheet, rather than processed by a
PivotTable or a chart), and Unpivot is the direct, purpose-built fix,
rather than something requiring manual copy-paste reshaping.

Notes:
- A tall Product/Month/Sales table like this is also exactly the
  shape a PivotTable (Excel Fundamentals Chapter 9, or this course's
  own Chapter 3) needs to build a Month-by-Product summary — the wide
  original layout couldn't be dragged into Rows/Columns/Values nearly
  as usefully.