A domino can knock over another domino about 1.5x larger than itself. A chain of dominos of increasing size makes a kind of mechanical chain reaction that starts with a tiny push and knocks down an impressively large domino.
Original idea by Lorne Whitehead, American Journal of Physics, Vol. 51, page 182 (1983).
for sophisticated discussions of the physics.
As seen on QI in the "kinetic" episode. I was contacted by those Elves.
Ron: This demo uses 13 dominos. The first domino is about 5 millimetres high and one millimetre thick. The last domino weighs about 100 pounds and is more than a metre tall. IF there were 29 dominos the last one would be as tall as the Empire State Building.