[plt-scheme] The Lambda Calculus behind functional programming

When I tell people that I'm learning Scheme the first thing they ask
me is "So have you learned lambda calculus?". I've learned enough to
say the words "lambda calculus" and that everyone says that lambda
calculus is the theoretical backbone of functional programming; but
that is it.

The following question is a simple one, it makes no attempt to
generalize whether anything is worth learning, I think you get the
idea.

So to revisit this again, what do you need to learn of the lambda
calculus relative to FP?

What would you tell folks about who have never looked at FP when they
ask you about lambda calculus?