jQuery - simple input validation - "empty" and "not empty"

I have an input that I want to validate:

<input type="text" id="input" />

And here's the JS:

        jQuery("#input").live('change', function() {

            if("#input:not(:empty)") {
                alert('not empty');
            }   

            else if("#input:empty") {
                alert('empty'); 
            }
        });

Even when the "#input" is empty there's no way of displaying alert with "empty" message. So, basically, no matter what I do, only the first statement is true and the second is false, always.

What's wrong?