Being able to construct logic gates is essential in order to create a computer. These logic gates can be created with just about anything — sticks and string, cogs and gears, even DNA. The only reason computers are electronic is because logic gates made from electronic switches can be turned on and off very quickly.
It turns out you can even build logic gates from dominos:
[youtube]http://www.youtube.com/watch?v=SudixyugiX4&eurl=http://www.mrreid.org[/youtube]
Ex-OR was genious but that guy has way too much time on his hands. Needs a EX-NOR, it would be huge.