Programmatically updating one update panel elements from another updat

While taking interviews for asp.net candidate I am often asking this question but most peoples are not able to give this answer. So I decided to write a blog post about this. Here is the scenario. There are two update panels in my html code in first update panel there is textbox hello world and another update panel there is a button called btnHelloWorld. Now I want to update textbox text in button click event without post back. But in normal scenario It will not update the textbox text as both are in dif...