Here's something I wonder:

Imagine you're stuck on an island or in a forest, and you're stuck with what you find in nature only, or perhaps you get very primitive technology (e.g. basic metal working only):

What's the simplest way you could build some (mechanical) logic gates to do some form of useful computation?

I've seen some from lego or 3D printed ones, but they all look very complicated, e.g. requiring rubber bands or springs but not looking practical to combine many together in a useful circuit