In a nutshell, the human brain,for most common people, can hold 7 different things at best. This is for common people, and this is an average. (I'm afraid I haven't found the source of my statement)

As a consequence, if you want to write code that anyone can read, it should not use more than 7 differents variables/concepts/functions/operations.

for example:

rotate(translate(rotate(translate(circle,50,0),0,10,pi),-50,0),0,10,pi)

there it is really hard to understand what is going on, astonishingly this lines does absolutely nothing in the geometric sense:

circle = translate(circle,50,0)
circle = rotate(circle,0,10,pi)
circle = translate(circle,-50,0)
circle = rotate(circle,0,10,-pi)

this is also why it is easier to read functions that are less than 40 lines of code too.