How I use java script for clear textbox value when click inside textbox in ASP.NET?

I am using asp.net textbox in one of web application, there 
<asp:TextBox ID="txtUserID" runat="server" Text="Email" CssClass="Sidebar_Login_txtUserID" ToolTip="Email" onClick="(this.value='')"></asp:TextBox> <asp:TextBox ID="txtPassword" runat="server" Text="password" ToolTip="Password" CssClass="Sidebar_Login_txtPassword" onClick="(this.value='')" TextMode="Password" ></asp:TextBox> in above coding display Username textbox display with 'Email' caption inside of the textbox.. when i click this one textbox text clear.. 
Now after release from textbox, If Textbox does not contain value this one display always empty without caption text.
So I want whenever dont have value it display the caption value ....

How i do it?



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