clearing textbox with onClick event

Hi guys..
I want to add following effects:
 
-When user clicks on ID textbox its contents should get cleared but password box should show Password if its content is already cleared.
 
-when user clicks on password box its contents should get cleared but Id text box should show show text now as ID ( so that user will get to know they have to enter ID in the text box)
 
I managed to clear the text box but once the text ID gets cleared i am not able to have it back when user clicks on password text box. 
 // THESE LINES I AM USING TO CLEAR BOXES

 txtId.Attributes.Add("onClick", "this.value='' ")

 txtpwd.Attributes.Add("onClick", "this.value='' ")
 
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