Conceptually simple things that are hard to get right

We’ve recently had a thread about leading a mercenary army back in time, and that along with previous threads about making black powder led me to research just how hard it is to actually make good quality gunpowder. Apparently there’s quite a bit more to it than simply mixing three ingredients together in the right proportions. Smokeless powder is far worse. I had imagined making it was as simple as cotton+nitric acid, and the ancient Romans could make nitric acid, so no problem, right? Wrong. Apparently it required the state of the art of late-ninetheenth century chemistry and industrial production for smokeless powders to be practical. The published descriptions of nuclear weapon design must surely gloss over immense technical and engineering headaches, meaning that nukes are NOT as simple as “bang two lumps of U-235 together”. I was curious what other things are nowhere near as easy to make as non-experts might naively suppose.

Recursion in computer programming, allegedly. People who get it tell me it’s simple, but I don’t get it.

I suck at music transposition. I don’t know if thats supposed to be easy or not, but I always forget how to transpose off of Bb parts to C parts.

sex. Five minutes to learn, a lifetime to master.

I always felt music fell into this category. In concept, it’s just playing particular notes at a particular time.

In practice, it’s really tough for a while, unless you have a natural knack for it.