How to move value from one form to another

 Hi All,
         In my .aspx page there is a button and a textbox, if I click on this button a new form open where i have bound some data. Now what i make it like if i click on any item on this newly open form then the value or text should fill in the first form's textbox.
should i use query string, if yes then how can u explain a bit
thanx

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