smib is one of the smallest and simplest computer algebra system in the world, but simple does not mean simplistic.
Using smib you can explore many branches of mathematics (e.g. number theory, algebra, calculus, numerical analysis, differential geometry, probability and stochastic calculus) and also some physics (e.g. electromagnetism and quantum mechanics)..
By simple you can understand easy to program (smib is a dynamically typed language, with strong affinity to recursivity, programs are often short and powerful and maybe useful for teaching), and also easy to modify (source code is free, written in C language, and based on notions of stacks and lists).
...