Object [object Object] has no method 'on'

I'm pretty new to backbone and I'm reading, following its example to create a todo list. I can't figure out why this piece of code does not work:

 var Todo= Backbone.Model.extend({
    defaults:       {
        title       :   "",
        completed   :   false
    },

    initialize:     function(){
            console.log("model initialized");
            this.on("change", function(){
                console.log("values for this model have changed.");
            });
    }        
});

var todo1= new Todo(); 

the libraries I've included are jquery, underscore and backbone. What's wrong with this? Why ".on" is not available? Thanks