We all have our hobbies. Some people paint miniatures. Others play tennis. A fellow who calls himself "lapinozz" likes to construct calculators out of marbles and cardboard. I know, it sounds like a bizarre episode of MacGyver, but this is a real thing.
A self-declared C++ programmer, lapinozz built this 4-bit computer "with [his] little sisters for a science activity", according to his blog. Called "LOGIC", which apparently stands for "Logic cardbOard Gates Inpredicable Calculator", the calculator isn't the most reliable gizmo and takes input in the form of binary:
...a marble is a one, no marble is a zero. The bit on the right is the least significant bit. You have to reset some part of the calculator before each calculation too. Then you remove the little bit of cardboard that kept the marbles from running down and they will slide along the paths, moving and changing the paths as they go, to finally arrive at the outputs at the bottom, giving you the result, as binary again.
It's unreliable simply because it's made from glue and cardboard. The marbles themselves can also be unpredictable:
Marbles would go too fast or too slow, they would bounce off and that sorts of things. This is not to say that it can’t work, the calculator work but you have to try multiple time before everything go the way it supposed to be. The logic itself is flawless, it’s just that cardboard turn out to not always be the most practical and require a lot of adjustment, we just couldn’t finish all the adjustment on time.
It can only add numbers between zero and 15 (hence it's 4-bit) and doesn't really stand a chance against an actual calculator... or even your fingers, for that matter. But as a fun project, it's really quite cool.
If you'd like to see it in action, lapinozz has some GIFs on his blog.