Challenge 1: Copy vs. Series with the Fill Handle — Solution Walkthrough

1. A1 = 2, A2 = 4. Select both A1:A2, drag the fill handle down to A10.
2. B1 = 2 only. Select just B1, drag its fill handle down to B10.

Expected result:

Column A (2, 4, dragged from a two-cell selection):
  2, 4, 6, 8, 10, 12, 14, 16, 18, 20

Column B (2, dragged from a one-cell selection):
  2, 2, 2, 2, 2, 2, 2, 2, 2, 2

Explanation:

With TWO cells selected (A1:A2), Excel has enough information to
detect a pattern — it sees 2, then 4, and infers the increment is
+2, so it continues the series: 6, 8, 10, and so on.

With only ONE cell selected (B1), Excel has no second data point to
compare against, so there's no increment to detect at all — it falls
back to its default behaviour for a lone value, which is to simply
COPY it unchanged into every cell you drag across.

Notes:
- This is exactly why this chapter's warning box calls out holding
  Ctrl while dragging a single number — Ctrl forces the +1 series
  behaviour even when only one cell (with no detectable pattern) is
  selected, overriding the "just copy it" default.
- The same two-cells-establish-a-pattern logic applies to dates, and
  to any custom list Excel has been taught (File > Options > Advanced
  > Edit Custom Lists) — one value copies, two values (or one value
  from a known list, like a weekday name) can imply a series.