Pleased with this one.
I think it is because they have a symmetry that we all understand, and when you draw them, if you get it even slightly off the eye sees it. Like a twist in a spine. You understand it without needing to analyse it.
The paper in this sketch book is very thin and my heavy handed shading can wreak havoc through the pages, as you can see.
But it was only £2 so I still love it.