Here’s a functional all-Lego microscope: https://ideas.lego.com/projects/fcce15cd-27e0-405b-990b-681b... No non-Lego pieces; even the lenses are Lego minifig-scale magnifying glasses. Unfortunately, it didn’t get enough backers in time to advance to production.
Lego seems to provide just the right amount creative control whilst still imposing its own rules and limitations. You see tiny models that express the character of the subject so well, often by using blocks in unexpected ways to create the subtlest hint of a form. The end result has so much charm but requires great skill.
That site led me to this working piano:
https://www.youtube.com/watch?v=dM7kj5XI_HY
Well, I guess it's silent, but it has the mechanics. Wow.