JQuery :: Simulate A Human-triggered Event?

I'm trying to change the value of an input field (target) which depends on another input (source). It works well when I manually change the source value. But if I changed the source value with another button, the target value remains the same.

Here's the code...

$(document).ready(function() {
$('input#change').click(function() {
$('input#source').attr('value', 'This is the value changed by a
button');

[Code].....

So how could the target input detect if there's a change within the source input without manually changing it's value?