Uncaught ReferenceError: angular is not defined - AngularJS not working

Question

I\'m attempting to learn angular and I am struggling with a simple button click.
I followed an example which has an identical code to the one below.

The result I

Answer 1

Just to complement m59's correct answer, here is a working jsfiddle:

    

        Click Me!
    
     
{{2+3}}