Use closures to create a simple object system. You'll see that you can quickly create systems ranging from Java-esque rigidity to smalltalk message passing to

Use closures to create a simple object system. You'll see that you can quickly create systems ranging from Java-esque rigidity to smalltalk message passing to javascript prototypes pretty quickly. Then you can start making your own trade offs in object systems. This is something most people (including lisp programmers) don't do daily, and it gets you wondering about some very abstract things. The imposed structure of say, Python (or most other languages), limits you to thinking about "only" the problem domain. For most people (again, including lispers), narrowing the mental focus helps a lot.