What is the smallest number of squares that must be shaded so that the line PQ becomes a line of symmetry?

1. Identify the line of symmetry and the positions of the currently shaded squares.

The line of symmetry $PQ$ is the diagonal from the bottom-left to the top-right of the $6 \times 6$ grid.
The currently shaded squares are at positions (Row 2, Col 4), (Row 3, Col 4), (Row 3, Col 5), (Row 4, Col 4), and (Row 4, Col 5).

2. Determine the reflection of each shaded square across the line of symmetry $PQ$.

Reflecting across the diagonal $PQ$ swaps the relative positions from the top-left and bottom-right. Specifically, a square at (Row $r$, Col $c$) reflects to (Row $7-c$, Col $7-r$):

• (Row 2, Col 4) reflects to (Row 3, Col 5), which is already shaded.
• (Row 3, Col 4) lies exactly on the diagonal $PQ$ ($3+4=7$), so it reflects to itself.
• (Row 4, Col 4) reflects to (Row 3, Col 3), which is currently unshaded.
• (Row 4, Col 5) reflects to (Row 2, Col 3), which is currently unshaded.

3. Count the number of additional squares that need to be shaded.

From the reflections, the squares at (Row 3, Col 3) and (Row 2, Col 3) must be shaded to make the figure symmetrical.
Number of squares to shade = $2$.