Note that greet_bob(say_hello) refers to two functions, but in different ways: greet_bob() and say_hello. The say_hello function is named without parentheses. This means that only a reference to the function is passed. The function is not executed. The greet_bob() function, on the other hand, is written with parentheses, so it will be called as usual.

After having shot a man in a Santa Fe bar, the famous artist Jim Stegner served his time and has since struggled to manage the dark impulses that sometimes overtake him.  Now he lives a quiet life. . . until the day that he comes across a hunting guide beating a small horse, and a brutal act of new violence rips his quiet life right open. Pursued by men dead set on retribution, Jim is left with no choice but to return to New Mexico and the high-profile life he left behind, where he’ll reckon with past deeds and the dark shadows in his own heart. 

Syntactic constraints. Python is a syntactically simple language with fairly strong constraints on what can and can't be done without "messing things up" (both visually and with regards to the language parser). There's no obvious way to structure this information so that people new to the concept will think, "Oh yeah, I know what you're doing." The best that seems possible is to keep new users from creating a wildly incorrect mental model of what the syntax means.
Color, made up of hue, saturation, and value, dispersed over a surface is the essence of painting, just as pitch and rhythm are the essence of music. Color is highly subjective, but has observable psychological effects, although these can differ from one culture to the next. Black is associated with mourning in the West, but in the East, white is. Some painters, theoreticians, writers and scientists, including Goethe,[3] Kandinsky,[4] and Newton,[5] have written their own color theory.