When the Emacs editor is started without an initial file to load, it creates a buffer called *scratch* in a special mode called Lisp Interaction Mode. This mode allows you to communicate directly with the lisp interpreter that is at the heart of Emacs.

To evaluate a lisp expression, enter it into the Lisp Interaction buffer and press C-j (control-J). Emacs will evaluate the expression and print out the results in the buffer. M-TAB (Esc then tab) will auto-complete lisp variables, or, if prefixed by an open-paranthesis, lisp functions.

This mode is useful for playing around with Emacs lisp. You can whip up quick lisp functions on the fly or do some quick math.