But falling dominoes are deceptively complex. A domino can slide against its neighbor after colliding, losing some energy to the friction. Alternatively, if there’s too little friction at the domino’s base, its bottom can slip out from underneath, and the domino will lose some of its forward momentum. For van Leeuwen’s equation to work simply, he needed to rule out those factors and imagine an ideal, purely mathematical domino that could avoid all these potential complications.

It turned out, van Leeuwen said, that an ideal domino could knock over a domino twice as tall, wide, and thick as itself — a growth factor of about 2 — so long as the dominoes were hollow. That means that while it would take about 20 solid dominoes with a growth factor of 1.5 to knock over a domino the size of the Domtoren, hollow dominoes that avoid friction could knock over a tower-sized brick in merely 12 steps.

thanks to insidescience.org for the great info