Challenge 1: FillSquares — Solution Walkthrough

Option Explicit

Sub FillSquares()
    Dim i As Integer
    For i = 1 To 10
        Cells(i, 1).Value = i ^ 2
    Next i
End Sub

Expected result:
  A1 = 1
  A2 = 4
  A3 = 9
  A4 = 16
  A5 = 25
  A6 = 36
  A7 = 49
  A8 = 64
  A9 = 81
  A10 = 100

WHY THIS WORKS AS AN ANSWER
------------------------------
Option Explicit sits as the very first line of the MODULE itself,
above the Sub statement — not inside the Sub — which is what forces
every variable in every procedure in that module to be declared
before use. The single Dim i As Integer declares the loop counter,
satisfying that requirement.

The For i = 1 To 10 loop runs its body exactly 10 times, with i
taking the values 1 through 10 in order, one per iteration. Cells(i,
1) addresses row i, column 1 (column A) — so as i counts up from 1 to
10, the code writes into A1 through A10 in turn. i ^ 2 is VBA's
exponentiation operator, squaring the current counter value before
it's stored.

Notes:
- Cells(i, 1) was deliberately used instead of Range("A" & i) (string-
  concatenating a cell address by hand) specifically because a
  numeric row/column pair is easier to work with inside a loop where
  the row itself is a variable, not fixed text — exactly the reason
  this chapter introduced Cells alongside Range in the first place.