Quaternions
Intuitive
see
https://plus.maths.org/content/os/issue32/features/baez/index
and the
excellent video on Quaternions
by 3Blue1Brown
The Strange Numbers That Birthed Modern Algebra
by Charlie Wood in Quanta Magazine.
Concrete
In this section things should be explained by analogy and with pictures and, if necessary, some formulas.
Abstract
The motto in this section is:
the higher the level of abstraction, the better
.
Why is it interesting?