To have your own head shrunk, launch Terminal.app, type "emacs" at the prompt (without the quotes, of course) then press Enter/Return.
The Unix text editor known as EMACS will load; hold down the Shift key and then press the Esc key. Type "xdoctor" (again, without the quotes), and press Enter.
Dr. Eliza should ask you a question now. Type a response, then press Enter twice. The doctor will ask another question. Type a response, then press Enter twice. And so on until you're sick of her. Control X, Control C will quit the emacs editor.