how to pass string value of child aspx to the parent aspx

Question

Hi i am linked the child aspx page with parent aspx page. when i click the button in parent aspx page the child aspx page opening. and when i click the OK button in the child aspx page, it has been closed no problem and now i want to pass the string value from the child aspx page (code behind) to parent aspx page textbox
 
How is possible?