A Tower of Dominoes

I noticed an open tin of wooden dominoes tipped over beside the rug. Instead of clearing them away, I balanced each rectangular block upright on its narrow edge, spacing them precisely an inch apart. It took controlled focus to keep my hands steady. When fifteen tiles stood in a tight curving arc, I gave the first piece a sharp tap with my fingertip. The rapid, clattering chain reaction snapped across the wood in a second, collapsing in a neat fan. Precision tested.