to clear Textbox onclick

Hi,

I have a textbox with a value (Enter Keywords)
what I want is when the user get to the page is to see the Enter Keyword
in the textbox and when the user clickon the text box it should clear the field.

Thanks,

Here's what I have.
 
<asp:TextBox ID="TxtKeywords" onkeydown="clearInput(this);" runat="server" Text="Enter Keywords"></asp:TextBox>
 
function clearInput(obj)
{
obj.value = ""; 
}

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