YouTube is chock full of falling domino videos, but Numberphile's Matt Parker may have trumped them all with a complicated 10,000 domino setup that just so happens to function as a very crude computer. How is such a thing even possible? This primer video explains the basics.
Depending on how you choose to start the chain reaction, the larger scale version is capable of adding any two four-digit binary numbers and returning the result as a five-digit binary sum. So while its calculating prowess isn't exactly mind-blowing, the fact that this even works (despite the setup running into a small calculating glitch) is pretty cool.