Grid Sorting

I moved across the carpet to the bottom shelf of the bookcase. Night had settled, and the space was quiet. Between two thick hardcovers, I found a pile of loose wooden domino tiles. They lay scattered in an uneven heap. I took the first tile and set it flat against the vertical wood panel. The second tile aligned flush with the edge of the first. I continued row by row, pressing each rectangular block into place until a flat, three-by-four grid covered the shelf. Every edge was squared. It made a solid barrier.