The spacing between dominoes controls how fast a topple travels — and closer is not always faster. Line up 100 dominoes at seven different gaps ranging from 10 mm to 40 mm, knock over the first tile, and time the full chain. Speed peaks at a 30 mm gap and drops on both sides. Too close, and each falling tile barely reaches the next. Too far apart, and each tile loses too much energy before triggering the one ahead. When each step needs just the right distance to trigger the next one, the chain runs fastest.

Wave propagation moves energy from one place to another without carrying matter along. You arrange 100 domino tiles in a row and time how long they take to topple over. Each tile stays in place, but the energy moves down the line from the first tile to the last.