[iliis]

What a fun question :) And there are some very cool answers!

A few things that come to mind from my life, from back when I was still a kid/teenager:

- Building a three-way marble-track switch. There are many marble tracks that have these two-way switches that alternate between two tracks, like this one here: https://theworkbenchutah.files.wordpress.com/2013/04/06-marb... I wanted to build one that would evenly distribute marbles along three tracks. Took me quite a few prototypes until I finally had a working one. And all I had to work with was paper, tape, glue etc. No wood or metal, no bearings or precision mechanics, only a kid's craft stuff.

- A LEGO technics robot leg. This is a bit hard to explain without images. I wanted to build a legged robot (something spider-like) and needed legs that could move a foot both borward and back and also up and down. As the LEGO motors where quite big and heavy I didn't want to mount them on the leg itself (e.g. at the "knee") but have them all fixed inside the body and transmit their power via gears and shaft. The problem here was, that the up-and-down-motion had to go trough the forward-and-back-joint which meant that whenever the leg moved forwards or backwards it would also move a bit up or down, even if the up-and-down-motor didn't move at all. So I wanted to decouple these two motions. This would be trivial to do in software but I was just a kid playing with LEGOs so I wanted a mechanical solution. I managed it by using a mechanical differential to actually subtract the one motion of the other. Unfortunately, I then realised I didn't have enough motors for a full robot...

- Years later I was learning C++ and I wanted some kind of "linked variables" (I'm sure there's an official term for this): I.e. variables that would depend on others and would update whenever one of their dependencies changed. I though this would be a cool way of writing a GUI. With a lot of operator overloading and some abuse of the type system I could actually write things like

    x = a + b * 2;
    y = x / b;

And 'y' would update whenever a or b (or x) was changed.

I'm not sure if they really are the hardest problems I've ever solved, but it certainly felt so at the time ;)

[_Nat_]

The "linked variables" concept is often called ["reactive programming"](https://en.wikipedia.org/wiki/Reactive_programming) or ["data binding"](https://en.wikipedia.org/wiki/Data_binding).

And you're right that it's very helpful with GUI specification.