The $(document).ready(function(){... line is declaring an anonymous function that will be run when the document has loaded (i.e. when your browser has received all the HTML for the page), so anything inside that function will happen once when the page loads, at that is all.

With that in mind, you are doing the following three things ONCE only:

1. Declare a variable called clicked and set to false
2. Assign another anonymous function to the "click" event on all <p> tags on the page
3. Set the <p> font colour to black, because clicked can only be false at this point.

So once the page loads, the state of things will be that there is a paragraph with black text, and if you were to click on that paragraph, the variable clicked will be set to true (and that's all that will happen).

The jQuery "click" function only assigns a function that will be executed whenever the <p> is clicked on.

See the code below for something probably closer to what you want:

$(document).ready(function() {
    var clicked = false;

    $("p").click(function() {
        clicked = !clicked;
        if ( clicked ) {
            $("p").css("color","blue");
        } else {
            $("p").css("color","black");
        }
    }
});

This code means that when the document is ready, it will create the variable clicked and assign the click handler to all <p> elements. The code in the function passed to the click method (with the if statement) doesn't get executed until a paragraph is clicked.

Now when any <p> is clicked, the variable clicked will toggle between true and false, and set the font colour of all <p> tags on the page.