How I explained to Lauren the difference between a function and its invocation

functions-and-function-invocations.md

$$ f(x) = x + 3 $$

y = x + 3

function f turns x into x + 3

we apply function f by saying f(something)

f = function (x) { return(x + 3) }

f   # gives us the dictionary definition of addThree 
f(something) # *applies* f to something

original text:

f(x) = x + 3

y = x + 3

function f turns x into x + 3
we apply function f by saying f(something)

function addThree

f
f()