You are given three grids, where each of them is 5 by 5 in size.
Grids have empty cells marked with "▢" and filled cells marked with "X".
Your task is to generate a referring expression that best describes the target grid while distinguishing it from the two other distractor grids.
The first grid is the target grid, and the following two grids are the distractors.

Target grid:

X X X ▢ X
X X X ▢ X
X X X ▢ X
X X X ▢ X
X X X ▢ X

Distractor grid 1:

▢ X X X X
▢ X X X X
▢ X X X X
▢ X X X X
▢ X X X X

Distractor grid 2:

X ▢ X X X
X ▢ X X X
X ▢ X X X
X ▢ X X X
X ▢ X X X

Instruction: Describe the target grid.
Generate the referring expression starting with the tag "Expression: " for the given target grid. Omit any other text.
Expression: The target grid has a unique pattern of four rows and five columns, where each row contains three X's followed by one empty cell (▢). This arrangement is distinct from the two distractor grids which have different patterns of filled and empty cells.
assistant
Expression: The grid with a 3x5 block of filled cells in the center, surrounded by a border of filled cells on the left, right, and top.
assistant