Clear Cascading DropDownList Selection when Parent DropDownList is changed

hi i am having two dropdownlist in my project,one dropdownlist for empid,another dropdownlist for month,,,if the user select the empid ,the values are retrived in the textbox ,,,in the second dropdownlist,if the user select the month in the dropdownlist it will display the leave details,its all working fine,,if the user again select the first dropdownlist that means empid,i want to show the second dropdownlist as select month,,,how can do this,,,,