Aha! moments in Lisp... What were your 'Aha moments' in your favorite language?

You get a similar feeling with Javascript by reading the source code of libraries like prototype.js or jQuery. Lots of "Wait, you can do that in Javascript?!?" moments.

Python: - the first time I concisely solved a seemingly complex problem with a few list comprehensions and a well thought out structure of maps and lists

C: - Using an include file as a simple database and redefining a macro to form a primitive code generator (first saw this in a CPU simulator)

Haskell: - SPJ's books on implementing functional languages -- the simple beginning pieces seemed to generate more Aha moments (pretty printing and parsing)

While none of these are specific to the given language, they represent points-of-no-return for a change in the way I think about solving (or even representing) problems.

My Aha moment was the first time I saw one program produce output which was the source code of a new program. Today we call that "code generation". Back then, I called it "the day everything changed". To this day, whenever I write something, I ask myself, "What could have generated that?"

Works well in any language.