Deriving the Y Combinator in Ruby

HN recommends it repeatedly as one of the best and most entertaining programming talks, and as the talk that makes the Y combinator click, even for people who do not use Ruby.

Jim Weirich's RubyConf 2012 talk derives the Y combinator step by step, live-coding in Ruby lambdas. It builds up an understanding of functional programming and lambda calculus from first principles.