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).
See
http://arxiv.org/abs/physics/0401018 and
http://arxiv.org/abs/1301.0615
for sophisticated discussions of the physics.
As seen on QI in the "kinetic" episode. I was contacted by those Elves.